diff --git a/sway-core/src/semantic_analysis/namespace/trait_map.rs b/sway-core/src/semantic_analysis/namespace/trait_map.rs
index 74d8b2ace33..feaeba38e99 100644
--- a/sway-core/src/semantic_analysis/namespace/trait_map.rs
+++ b/sway-core/src/semantic_analysis/namespace/trait_map.rs
@@ -237,7 +237,7 @@ impl TraitMap {
         is_extending_existing_impl: IsExtendingExistingImpl,
         engines: &Engines,
     ) -> Result<(), ErrorEmitted> {
-        let type_id = engines.te().get_unaliased_type_id(type_id);
+        let unaliased_type_id = engines.te().get_unaliased_type_id(type_id);
 
         handler.scope(|handler| {
             let mut trait_items: TraitItems = HashMap::new();
@@ -267,7 +267,7 @@ impl TraitMap {
                 }
             }
 
-            let trait_impls = self.get_impls_mut(engines, type_id);
+            let trait_impls = self.get_impls_mut(engines, unaliased_type_id);
 
             // check to see if adding this trait will produce a conflicting definition
             for TraitEntry {
@@ -295,11 +295,11 @@ impl TraitMap {
 
                 let unify_checker = UnifyCheck::non_generic_constraint_subset(engines);
 
-                // Types are subset if the `type_id` that we want to insert can unify with the
+                // Types are subset if the `unaliased_type_id` that we want to insert can unify with the
                 // existing `map_type_id`. In addition we need to additionally check for the case of
                 // `&mut <type>` and `&<type>`.
-                let types_are_subset = unify_checker.check(type_id, *map_type_id)
-                    && is_unified_type_subset(engines.te(), type_id, *map_type_id);
+                let types_are_subset = unify_checker.check(unaliased_type_id, *map_type_id)
+                    && is_unified_type_subset(engines.te(), unaliased_type_id, *map_type_id);
 
                 /// `left` can unify into `right`. Additionally we need to check subset condition in case of
                 /// [TypeInfo::Ref] types.  Although `&mut <type>` can unify with `&<type>`
@@ -368,6 +368,9 @@ impl TraitMap {
                     handler.emit_err(CompileError::ConflictingImplsForTraitAndType {
                         trait_name: trait_name.to_string_with_args(engines, &trait_type_args),
                         type_implementing_for: engines.help_out(type_id).to_string(),
+                        type_implementing_for_unaliased: engines
+                            .help_out(unaliased_type_id)
+                            .to_string(),
                         existing_impl_span: existing_impl_span.clone(),
                         second_impl_span: impl_span.clone(),
                     });
@@ -382,7 +385,7 @@ impl TraitMap {
                             ResolvedTraitImplItem::Parsed(_item) => todo!(),
                             ResolvedTraitImplItem::Typed(item) => match item {
                                 ty::TyTraitItem::Fn(decl_ref) => {
-                                    if map_trait_items.get(name).is_some() {
+                                    if let Some(existing_item) = map_trait_items.get(name) {
                                         handler.emit_err(
                                             CompileError::DuplicateDeclDefinedForType {
                                                 decl_kind: "method".into(),
@@ -390,13 +393,19 @@ impl TraitMap {
                                                 type_implementing_for: engines
                                                     .help_out(type_id)
                                                     .to_string(),
-                                                span: decl_ref.name().span(),
+                                                type_implementing_for_unaliased: engines
+                                                    .help_out(unaliased_type_id)
+                                                    .to_string(),
+                                                existing_impl_span: existing_item
+                                                    .span(engines)
+                                                    .clone(),
+                                                second_impl_span: decl_ref.name().span(),
                                             },
                                         );
                                     }
                                 }
                                 ty::TyTraitItem::Constant(decl_ref) => {
-                                    if map_trait_items.get(name).is_some() {
+                                    if let Some(existing_item) = map_trait_items.get(name) {
                                         handler.emit_err(
                                             CompileError::DuplicateDeclDefinedForType {
                                                 decl_kind: "constant".into(),
@@ -404,13 +413,19 @@ impl TraitMap {
                                                 type_implementing_for: engines
                                                     .help_out(type_id)
                                                     .to_string(),
-                                                span: decl_ref.name().span(),
+                                                type_implementing_for_unaliased: engines
+                                                    .help_out(unaliased_type_id)
+                                                    .to_string(),
+                                                existing_impl_span: existing_item
+                                                    .span(engines)
+                                                    .clone(),
+                                                second_impl_span: decl_ref.name().span(),
                                             },
                                         );
                                     }
                                 }
                                 ty::TyTraitItem::Type(decl_ref) => {
-                                    if map_trait_items.get(name).is_some() {
+                                    if let Some(existing_item) = map_trait_items.get(name) {
                                         handler.emit_err(
                                             CompileError::DuplicateDeclDefinedForType {
                                                 decl_kind: "type".into(),
@@ -418,7 +433,13 @@ impl TraitMap {
                                                 type_implementing_for: engines
                                                     .help_out(type_id)
                                                     .to_string(),
-                                                span: decl_ref.name().span(),
+                                                type_implementing_for_unaliased: engines
+                                                    .help_out(unaliased_type_id)
+                                                    .to_string(),
+                                                existing_impl_span: existing_item
+                                                    .span(engines)
+                                                    .clone(),
+                                                second_impl_span: decl_ref.name().span(),
                                             },
                                         );
                                     }
@@ -442,7 +463,7 @@ impl TraitMap {
                 trait_name,
                 impl_span.clone(),
                 trait_decl_span,
-                type_id,
+                unaliased_type_id,
                 trait_items,
                 engines,
             );
diff --git a/sway-error/src/error.rs b/sway-error/src/error.rs
index 349ac2fc7a3..7a085ae7745 100644
--- a/sway-error/src/error.rs
+++ b/sway-error/src/error.rs
@@ -625,6 +625,7 @@ pub enum CompileError {
     ConflictingImplsForTraitAndType {
         trait_name: String,
         type_implementing_for: String,
+        type_implementing_for_unaliased: String,
         existing_impl_span: Span,
         second_impl_span: Span,
     },
@@ -640,7 +641,9 @@ pub enum CompileError {
         decl_kind: String,
         decl_name: String,
         type_implementing_for: String,
-        span: Span,
+        type_implementing_for_unaliased: String,
+        existing_impl_span: Span,
+        second_impl_span: Span,
     },
     #[error("The function \"{fn_name}\" in {interface_name} is defined with {num_parameters} parameters, but the provided implementation has {provided_parameters} parameters.")]
     IncorrectNumberOfInterfaceSurfaceFunctionParameters {
@@ -1180,7 +1183,9 @@ impl Spanned for CompileError {
                 second_impl_span, ..
             } => second_impl_span.clone(),
             MarkerTraitExplicitlyImplemented { span, .. } => span.clone(),
-            DuplicateDeclDefinedForType { span, .. } => span.clone(),
+            DuplicateDeclDefinedForType {
+                second_impl_span, ..
+            } => second_impl_span.clone(),
             IncorrectNumberOfInterfaceSurfaceFunctionParameters { span, .. } => span.clone(),
             ArgumentParameterTypeMismatch { span, .. } => span.clone(),
             RecursiveCall { span, .. } => span.clone(),
@@ -2317,20 +2322,30 @@ impl ToDiagnostic for CompileError {
                     format!("{}- referencing a mutable copy of \"{decl_name}\", by returning it from a block: `&mut {{ {decl_name} }}`.", Indent::Single)
                 ],
             },
-            ConflictingImplsForTraitAndType { trait_name, type_implementing_for, existing_impl_span, second_impl_span } => Diagnostic {
+            ConflictingImplsForTraitAndType { trait_name, type_implementing_for, type_implementing_for_unaliased, existing_impl_span, second_impl_span } => Diagnostic {
                 reason: Some(Reason::new(code(1), "Trait is already implemented for type".to_string())),
                 issue: Issue::error(
                     source_engine,
                     second_impl_span.clone(),
-                    format!("Trait \"{trait_name}\" is already implemented for type \"{type_implementing_for}\".")
+                    if type_implementing_for == type_implementing_for_unaliased {
+                        format!("Trait \"{trait_name}\" is already implemented for type \"{type_implementing_for}\".")
+                    } else {
+                        format!("Trait \"{trait_name}\" is already implemented for type \"{type_implementing_for}\" (which is an alias for \"{type_implementing_for_unaliased}\").")
+                    }
                 ),
                 hints: vec![
                     Hint::info(
                         source_engine,
                         existing_impl_span.clone(),
-                        format!("This is the already existing implementation of \"{}\" for \"{type_implementing_for}\".",
-                            call_path_suffix_with_args(trait_name)
-                        )
+                        if type_implementing_for == type_implementing_for_unaliased {
+                            format!("This is the already existing implementation of \"{}\" for \"{type_implementing_for}\".",
+                                    call_path_suffix_with_args(trait_name)
+                            )
+                        } else {
+                            format!("This is the already existing implementation of \"{}\" for \"{type_implementing_for}\" (which is an alias for \"{type_implementing_for_unaliased}\").",
+                                    call_path_suffix_with_args(trait_name)
+                            )
+                        }
                     ),
                 ],
                 help: vec![
@@ -2338,6 +2353,31 @@ impl ToDiagnostic for CompileError {
                     "This property is called \"trait coherence\".".to_string(),
                 ],
             },
+            DuplicateDeclDefinedForType { decl_kind, decl_name, type_implementing_for, type_implementing_for_unaliased, existing_impl_span, second_impl_span } => {
+                let decl_kind_snake_case = sway_types::style::to_upper_camel_case(decl_kind);
+                Diagnostic {
+                    reason: Some(Reason::new(code(1), "Type contains duplicate declarations".to_string())),
+                    issue: Issue::error(
+                        source_engine,
+                        second_impl_span.clone(),
+                        if type_implementing_for == type_implementing_for_unaliased {
+                            format!("{decl_kind_snake_case} \"{decl_name}\" already declared in type \"{type_implementing_for}\".")
+                        } else {
+                            format!("{decl_kind_snake_case} \"{decl_name}\" already declared in type \"{type_implementing_for}\" (which is an alias for \"{type_implementing_for_unaliased}\").")
+                        }
+                    ),
+                    hints: vec![
+                        Hint::info(
+                            source_engine,
+                            existing_impl_span.clone(),
+                            format!("\"{decl_name}\" previously defined here.")
+                        )
+                    ],
+                    help: vec![
+                        "A type may not contain two or more declarations of the same name".to_string(),
+                    ],
+                }
+            },
             MarkerTraitExplicitlyImplemented { marker_trait_full_name, span} => Diagnostic {
                 reason: Some(Reason::new(code(1), "Marker traits cannot be explicitly implemented".to_string())),
                 issue: Issue::error(
diff --git a/test/src/e2e_vm_tests/test_programs/should_fail/impl_self_duplicated/test.toml b/test/src/e2e_vm_tests/test_programs/should_fail/impl_self_duplicated/test.toml
index 716219cfde9..5b27cecd833 100644
--- a/test/src/e2e_vm_tests/test_programs/should_fail/impl_self_duplicated/test.toml
+++ b/test/src/e2e_vm_tests/test_programs/should_fail/impl_self_duplicated/test.toml
@@ -2,5 +2,9 @@ category = "fail"
 validate_abi = false
 expected_warnings = 3
 
-# check: fn foo(self) -> u64 {
-# nextln: $()Duplicate definitions for the method "foo" for type "S<T>".
+# check: $()Type contains duplicate declarations
+# check: $()fn foo(self) -> u64 {
+# check: $()"foo" previously defined here.
+# check: $()Method "foo" already declared in type "S<T>".
+
+# check: $()Aborting due to 1 error.
\ No newline at end of file
diff --git a/test/src/e2e_vm_tests/test_programs/should_fail/multiple_methods_with_the_same_name/test.toml b/test/src/e2e_vm_tests/test_programs/should_fail/multiple_methods_with_the_same_name/test.toml
index 7c383bbf1db..2e4be8b0f3d 100644
--- a/test/src/e2e_vm_tests/test_programs/should_fail/multiple_methods_with_the_same_name/test.toml
+++ b/test/src/e2e_vm_tests/test_programs/should_fail/multiple_methods_with_the_same_name/test.toml
@@ -1,13 +1,27 @@
 category = "fail"
 
+# check: $()Type contains duplicate declarations
 # check: $()fn my_add(self, other: Self) -> Self {
-# check: $()Duplicate definitions for the method "my_add" for type "Data<T>".
+# check: $()"my_add" previously defined here.
+# check: $()fn my_add(self, other: Self) -> Self {
+# check: $()Method "my_add" already declared in type "Data<T>".
 
+# check: $()Type contains duplicate declarations
+# check: $()fn get_value(self) -> u64 {
+# check: $()"get_value" previously defined here.
 # check: $()fn get_value(self) -> u64 {
-# check: $()Duplicate definitions for the method "get_value" for type "Data<u64>".
+# check: $(Method "get_value" already declared in type "Data<u64>".
 
+# check: $()Type contains duplicate declarations
+# check: $()fn my_add(self, other: Self) -> Self {
+# check: $()"my_add" previously defined here.
 # check: $()fn my_add(self, other: Self) -> Self {
-# check: $()Duplicate definitions for the method "my_add" for type "Data<u64>".
+# check: $()Method "my_add" already declared in type "Data<u64>".
 
+# check: $()Type contains duplicate declarations
 # check: $()fn my_add(self, other: Self) -> Self {
-# check: $()Duplicate definitions for the method "my_add" for type "Data<u32>".
+# check: $()"my_add" previously defined here.
+# check: $()fn my_add(self, other: Self) -> Self {
+# check: $()Method "my_add" already declared in type "Data<u64>".
+
+# check: $()Aborting due to 6 errors
\ No newline at end of file
diff --git a/test/src/e2e_vm_tests/test_programs/should_fail/type_alias_shadowing_methods/test.toml b/test/src/e2e_vm_tests/test_programs/should_fail/type_alias_shadowing_methods/test.toml
index dde5c71514e..c24b2329807 100644
--- a/test/src/e2e_vm_tests/test_programs/should_fail/type_alias_shadowing_methods/test.toml
+++ b/test/src/e2e_vm_tests/test_programs/should_fail/type_alias_shadowing_methods/test.toml
@@ -1,5 +1,7 @@
 category = "fail"
 
-#check: $()impl Alias2 {
-#nextln: $()fn foo() {}
-#nextln: $()Duplicate definitions for the method "foo" for type "MyStruct1".
+#check: $()Type contains duplicate declarations
+#check: $()fn foo() {}
+#nextln: $()"foo" previously defined here.
+#check: $()fn foo() {}
+#nextln: $()Method "foo" already declared in type "Alias2" (which is an alias for "MyStruct1").
diff --git a/test/src/e2e_vm_tests/test_programs/should_fail/type_alias_unification/Forc.lock b/test/src/e2e_vm_tests/test_programs/should_fail/type_alias_unification/Forc.lock
new file mode 100644
index 00000000000..1f8113a3e20
--- /dev/null
+++ b/test/src/e2e_vm_tests/test_programs/should_fail/type_alias_unification/Forc.lock
@@ -0,0 +1,13 @@
+[[package]]
+name = "core"
+source = "path+from-root-6E306998EEBF0DBD"
+
+[[package]]
+name = "std"
+source = "path+from-root-6E306998EEBF0DBD"
+dependencies = ["core"]
+
+[[package]]
+name = "type_alias_unification"
+source = "member"
+dependencies = ["std"]
diff --git a/test/src/e2e_vm_tests/test_programs/should_fail/type_alias_unification/Forc.toml b/test/src/e2e_vm_tests/test_programs/should_fail/type_alias_unification/Forc.toml
new file mode 100644
index 00000000000..c3ac933766e
--- /dev/null
+++ b/test/src/e2e_vm_tests/test_programs/should_fail/type_alias_unification/Forc.toml
@@ -0,0 +1,8 @@
+[project]
+authors = ["Fuel Labs <contact@fuel.sh>"]
+entry = "main.sw"
+license = "Apache-2.0"
+name = "type_alias_unification"
+
+[dependencies]
+std = { path = "../../../reduced_std_libs/sway-lib-std-assert" }
diff --git a/test/src/e2e_vm_tests/test_programs/should_fail/type_alias_unification/src/main.sw b/test/src/e2e_vm_tests/test_programs/should_fail/type_alias_unification/src/main.sw
new file mode 100644
index 00000000000..36870f342e1
--- /dev/null
+++ b/test/src/e2e_vm_tests/test_programs/should_fail/type_alias_unification/src/main.sw
@@ -0,0 +1,32 @@
+script;
+
+trait MyTrait {
+    fn extract_a(self) -> u64;
+}
+
+struct A {
+    a: u64,
+}
+
+impl MyTrait for A {
+    fn extract_a(self) -> u64 {
+        self.a
+    }
+}
+
+type B = A;
+
+// B is an alias for A, and A already has an implementation of MyTrait,
+// so this should cause a compilation error.
+impl MyTrait for B {
+    fn extract_a(self) -> u64 {
+        self.a + 1
+    }
+}
+
+fn main() {
+    let struct_a = A { a: 1 }; 
+    let struct_b = B { a: 42 };
+    assert(struct_a.extract_a() == 1);
+    assert(struct_b.extract_a() == 42);
+}
diff --git a/test/src/e2e_vm_tests/test_programs/should_fail/type_alias_unification/test.toml b/test/src/e2e_vm_tests/test_programs/should_fail/type_alias_unification/test.toml
new file mode 100644
index 00000000000..ef322b7f74e
--- /dev/null
+++ b/test/src/e2e_vm_tests/test_programs/should_fail/type_alias_unification/test.toml
@@ -0,0 +1,7 @@
+category = "fail"
+
+#check: $()Trait is already implemented for type
+#check: $()This is the already existing implementation of "MyTrait" for "B" (which is an alias for "A").
+#check: $()Trait "type_alias_unification::MyTrait" is already implemented for type "B" (which is an alias for "A").
+
+#check: $()Aborting due to 1 error.