Improve branch combining logic for AggregateMeasuresNode

metricflow/dataflow/optimizer/source_scan/cm_branch_combiner.py

@@ -239,25 +239,27 @@ def visit_aggregate_measures_node(  # noqa: D102
 
         assert len(combined_parent_nodes) == 1
         combined_parent_node = combined_parent_nodes[0]
-        assert combined_parent_node is not None
 
         combined_metric_input_measure_specs = tuple(
             dict.fromkeys(
                 self._current_left_node.metric_input_measure_specs + current_right_node.metric_input_measure_specs
             ).keys()
         )
 
+        # Avoid combining branches if the AggregateMeasuresNode specifies a metric with an alias to avoid
+        # collisions e.g. two metrics use the same alias for two different measures. This is not always the case,
+        # so this could be improved later.
+        seen_aliases = set()
         for spec in combined_metric_input_measure_specs:
-            # Avoid combining branches if the AggregateMeasuresNode specifies a metric with an alias to avoid
-            # collisions e.g. two metrics use the same alias for two different measures. This is not always the case,
-            # so this could be improved later.
             if spec.alias is not None:
-                self._log_combine_failure(
-                    left_node=self._current_left_node,
-                    right_node=current_right_node,
-                    combine_failure_reason=f"Metric input measure spec {spec} has an alias",
-                )
-                return ComputeMetricsBranchCombinerResult()
+                if spec.alias in seen_aliases:
+                    self._log_combine_failure(
+                        left_node=self._current_left_node,
+                        right_node=current_right_node,
+                        combine_failure_reason=f"Found multiple metric input measure specs with alias '{spec.alias}'",
+                    )
+                    return ComputeMetricsBranchCombinerResult()
+                seen_aliases.add(spec.alias)
 
         combined_node = AggregateMeasuresNode.create(
             parent_node=combined_parent_node,