fix(rust): Ensure deduced join key names are unique (#16551)

crates/polars-plan/src/logical_plan/conversion/dsl_to_ir.rs

@@ -325,11 +325,6 @@ pub fn to_alp_impl(
                 }
             }
 
-            let mut joined_on = PlHashSet::new();
-            for (l, r) in left_on.iter().zip(right_on.iter()) {
-                polars_ensure!(joined_on.insert((l, r)), InvalidOperation: "joins on same keys twice; already joined on {} and {}", l, r)
-            }
-            drop(joined_on);
             options.args.validation.is_valid_join(&options.args.how)?;
 
             polars_ensure!(
@@ -356,6 +351,16 @@ pub fn to_alp_impl(
 
             let left_on = to_expr_irs_ignore_alias(left_on, expr_arena);
             let right_on = to_expr_irs_ignore_alias(right_on, expr_arena);
+            let mut joined_on = PlHashSet::new();
+            for (l, r) in left_on.iter().zip(right_on.iter()) {
+                polars_ensure!(
+                    joined_on.insert((l.output_name(), r.output_name())),
+                    InvalidOperation: "joining with repeated key names; already joined on {} and {}",
+                    l.output_name(),
+                    r.output_name()
+                )
+            }
+            drop(joined_on);
 
             convert.fill_scratch(&left_on, expr_arena);
             convert.fill_scratch(&right_on, expr_arena);

py-polars/tests/unit/operations/test_join.py

@@ -1014,3 +1014,13 @@ def test_left_join_coalesce_default_deprecation_message() -> None:
     right = pl.DataFrame({"a": [2, 3, 4], "c": [4, 5, 6]})
     with pytest.deprecated_call():
         left.join(right, on="a", how="left")
+
+
+@pytest.mark.parametrize("coalesce", [False, True])
+def test_join_raise_on_repeated_expression_key_names(coalesce: bool) -> None:
+    left = pl.DataFrame({"a": [1, 2, 3], "b": [3, 4, 5], "c": [5, 6, 7]})
+    right = pl.DataFrame({"a": [2, 3, 4], "c": [4, 5, 6]})
+    with pytest.raises(pl.InvalidOperationError, match="already joined on"):
+        left.join(
+            right, on=[pl.col("a"), pl.col("a") % 2], how="full", coalesce=coalesce
+        )