BUG: Join with list of dataframes that have MultiIndex now behaves as expected (pandas-dev#57676)

Author: Dacops

pandas/core/frame.py

@@ -10604,7 +10604,12 @@ def join(
             # "Iterable[Union[DataFrame, Series]]" due to the if statements
             frames = [cast("DataFrame | Series", self)] + list(other)
 
-            can_concat = all(df.index.is_unique for df in frames)
+            can_concat = True
+            for df in frames:
+                if isinstance(df.index, MultiIndex):
+                    can_concat = all(idx.is_unique for idx in df.index.levels)
+                elif isinstance(df.index, Index) and not df.index.has_duplicates:
+                    can_concat = False
 
             # join indexes only using concat
             if can_concat:

pandas/tests/frame/methods/test_join.py

@@ -562,3 +562,21 @@ def test_frame_join_tzaware(self):
 
         tm.assert_index_equal(result.index, expected)
         assert result.index.tz.zone == "US/Central"
+
+    def test_join_list_with_multiindex(self):
+        test1 = DataFrame(
+            {"cat": pd.Categorical(["a", "v", "d"])},
+            index=Index(["a", "b", "c"], name="y"),
+        )
+        test2 = DataFrame(
+            {"foo": np.arange(6)},
+            index=MultiIndex.from_tuples(
+                [(0, "a"), (0, "b"), (0, "c"), (1, "a"), (1, "b"), (1, "c")],
+                names=("x", "y"),
+            ),
+        )
+
+        result = test2.join([test1])
+        expected = test2.join(test1)
+
+        assert result.equals(expected)