chore: Don't raise on multiple same names in ie_join (#18658)

crates/polars-plan/src/plans/conversion/join.rs

@@ -90,13 +90,20 @@ pub fn resolve_join(
     let left_on = to_expr_irs_ignore_alias(left_on, ctxt.expr_arena)?;
     let right_on = to_expr_irs_ignore_alias(right_on, ctxt.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()
-        )
+
+    #[cfg(feature = "iejoin")]
+    let check = !matches!(options.args.how, JoinType::IEJoin(_));
+    #[cfg(not(feature = "iejoin"))]
+    let check = true;
+    if check {
+        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);
 

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

@@ -466,3 +466,19 @@ def test_raise_invalid_input_join_where() -> None:
     df = pl.DataFrame({"id": [1, 2]})
     with pytest.raises(pl.exceptions.InvalidOperationError):
         df.join_where(df)
+
+
+def test_ie_join_use_keys_multiple() -> None:
+    a = pl.LazyFrame({"a": [1, 2, 3], "x": [7, 2, 1]})
+    b = pl.LazyFrame({"b": [2, 2, 2], "x": [7, 1, 3]})
+
+    assert a.join_where(
+        b,
+        pl.col.a >= pl.col.b,
+        pl.col.a <= pl.col.b,
+    ).collect().sort("x_right").to_dict(as_series=False) == {
+        "a": [2, 2, 2],
+        "x": [2, 2, 2],
+        "b": [2, 2, 2],
+        "x_right": [1, 3, 7],
+    }