v2: check model ID uniqueness only in validate

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: FFroehlich

petab/v2/core.py

@@ -2123,14 +2123,11 @@ def split_by_model(self) -> dict[str, Problem]:
         See https://github.com/PEtab-dev/libpetab-python/issues/392.
 
         :returns: The single-model problems, keyed by model ID.
-        :raises ValueError: If model IDs are not unique, or if any
-            measurement is not assigned to a model.
+        :raises ValueError: If any measurement is not assigned to a model.
         """
         from ..v2.lint import get_valid_parameters_for_parameter_table
 
         model_ids = {model.model_id for model in self.models}
-        if len(model_ids) != len(self.models):
-            raise ValueError("Model IDs must be unique.")
 
         def get_model_id(measurement: Measurement) -> str | None:
             # with a single model, the model ID is optional

tests/v2/test_core.py

@@ -1049,12 +1049,6 @@ def test_split_by_model():
     with pytest.raises(ValueError, match="not assigned"):
         problem.split_by_model()
 
-    # duplicate model IDs
-    problem = _two_model_problem()
-    problem.models[1].model_id = "model_A"
-    with pytest.raises(ValueError, match="unique"):
-        problem.split_by_model()
-
 
 def test_split_by_model_single_model():
     """With one model, measurements don't need a model ID."""
@@ -1087,7 +1081,15 @@ def test_validate_multi_model():
     assert result.has_errors()
     assert "does not have a model ID" in result[0].message
 
+    # duplicate model IDs
+    problem = _two_model_problem()
+    problem.models[1].model_id = "model_A"
+    result = problem.validate()
+    assert result.has_errors()
+    assert "Duplicate model IDs" in result[0].message
+
     # unknown model ID
+    problem = _two_model_problem()
     problem.measurements[0].model_id = "model_C"
     result = problem.validate()
     assert result.has_errors()