diff --git a/modelskill/matching.py b/modelskill/matching.py
index 82cb58841..a00e8ed88 100644
--- a/modelskill/matching.py
+++ b/modelskill/matching.py
@@ -1,6 +1,5 @@
 from __future__ import annotations
 
-import warnings
 from datetime import timedelta
 from pathlib import Path
 from typing import (
@@ -361,7 +360,7 @@ def match_space_time(
 
     observation = observation.trim(period.start, period.end)
 
-    data = observation.data
+    data = observation.data.copy()
     data.attrs["name"] = observation.name
     data = data.rename({observation.name: "Observation"})
 
@@ -369,11 +368,12 @@ def match_space_time(
         # TODO is `align` the correct name for this operation?
         aligned = mr.align(observation, max_gap=max_model_gap)
 
-        if overlapping_names := set(aligned.data_vars) & set(data.data_vars):
-            warnings.warn(
-                "Model result has overlapping variable names with observation. Renamed with suffix `_model`."
+        if overlapping := set(aligned.filter_by_attrs(kind="aux").data_vars) & set(
+            observation.data.filter_by_attrs(kind="aux").data_vars
+        ):
+            raise ValueError(
+                f"Aux variables are not allowed to have identical names. Choose either aux from obs or model. Overlapping: {overlapping}"
             )
-            aligned = aligned.rename({v: f"{v}_mod" for v in overlapping_names})
 
         for dv in aligned:
             data[dv] = aligned[dv]
diff --git a/tests/test_match.py b/tests/test_match.py
index 5a2fe3c87..67cc873e1 100644
--- a/tests/test_match.py
+++ b/tests/test_match.py
@@ -444,10 +444,8 @@ def test_obs_and_mod_can_not_have_same_aux_item_names():
     obs = ms.PointObservation(obs_df, item="wl", aux_items=["wind_speed"])
     mod = ms.PointModelResult(mod_df, item="wl", aux_items=["wind_speed"])
 
-    with pytest.warns(match="_model"):
-        cmp = ms.match(obs=obs, mod=mod)
-    assert "wind_speed" in cmp
-    assert "wind_speed_mod" in cmp  # renamed
+    with pytest.raises(ValueError, match="aux"):
+        ms.match(obs=obs, mod=mod)
 
 
 def test_mod_aux_items_overlapping_names():
@@ -474,7 +472,7 @@ def test_mod_aux_items_overlapping_names():
         mod2_df, item="wl", aux_items=["wind_speed"], name="remote"
     )
 
-    # we don't care which model the aux data comes from
+    # we don't care which model the aux data comes from, last one wins
     cmp = ms.match(obs=obs, mod=[mod, mod2])
 
     assert "wind_speed" in cmp