Add terraform_map_duplicate_keys rule doc

Author: wata727

docs/rules/README.md

@@ -13,6 +13,7 @@ All rules are enabled by default, but by setting `preset = "recommended"`, you c
 |[terraform_documented_outputs](terraform_documented_outputs.md)|Disallow `output` declarations without description||
 |[terraform_documented_variables](terraform_documented_variables.md)|Disallow `variable` declarations without description||
 |[terraform_empty_list_equality](terraform_empty_list_equality.md)|Disallow comparisons with `[]` when checking if a collection is empty|✔|
+|[terraform_map_duplicate_keys](terraform_map_duplicate_keys.md)|Disallow duplicate keys in a map object|✔|
 |[terraform_module_pinned_source](terraform_module_pinned_source.md)|Disallow specifying a git or mercurial repository as a module source without pinning to a version|✔|
 |[terraform_module_version](terraform_module_version.md)|Checks that Terraform modules sourced from a registry specify a version|✔|
 |[terraform_naming_convention](terraform_naming_convention.md)|Enforces naming conventions for resources, data sources, etc||

docs/rules/terraform_map_duplicate_keys.md

@@ -0,0 +1,39 @@
+# terraform_map_duplicate_keys
+
+Disallow duplicate keys in a map object.
+
+> This rule is enabled by "recommended" preset.
+
+## Example
+
+```hcl
+locals {
+  map = {
+    foo = 1
+    bar = 2
+    bar = 3 // duplicate key
+  }
+}
+```
+
+```
+$ tflint
+1 issue(s) found:
+
+Warning: Duplicate key: "bar", first defined at main.tf:4,5-8 (terraform_map_duplicate_keys)
+
+  on main.tf line 5:
+   5:     bar = 3 // duplicated key
+
+Reference: https://github.com/terraform-linters/tflint-ruleset-terraform/blob/v0.9.0/docs/rules/terraform_map_duplicate_keys.md
+```
+
+## Why
+
+In the Terraform language, duplicate map keys are overwritten rather than throwing an error. However, in most cases this behavior is not what you want and is often caused by a mistake. This rule will catch such mistakes early.
+
+See also https://github.com/hashicorp/terraform/issues/28727
+
+## How To Fix
+
+Remove the duplicate keys and leave the correct value.