use old logic on duplicates in '.concat()'

Signed-off-by: Dmitry Chigarev <[email protected]>

modin/core/dataframe/pandas/metadata/dtypes.py

@@ -648,7 +648,16 @@ def concat(cls, values: list) -> "ModinDtypes":
             else:
                 raise NotImplementedError(type(val))
 
-        desc = DtypesDescriptor.concat(preprocessed_vals)
+        try:
+            desc = DtypesDescriptor.concat(preprocessed_vals)
+        except NotImplementedError as e:
+            # 'DtypesDescriptor' doesn't support duplicated labels, however, if all values are pandas Serieses,
+            # we still can perform concatenation using pure pandas
+            if "duplicated" not in e.args[0].lower() or not all(
+                isinstance(val, pandas.Series) for val in values
+            ):
+                raise e
+            desc = pandas.concat(values)
         return ModinDtypes(desc)
 
     def set_index(self, new_index: Union[pandas.Index, "ModinIndex"]) -> "ModinDtypes":

modin/test/storage_formats/pandas/test_internals.py

@@ -1815,6 +1815,24 @@ def test_concat(self):
         )
         assert res.equals(exp)
 
+    def test_ModinDtypes_duplicated_concat(self):
+        # test that 'ModinDtypes' is able to perform dtypes concatenation on duplicated labels
+        # if all of them are Serieses
+        res = ModinDtypes.concat([pandas.Series([np.dtype(int)], index=["a"])] * 2)
+        assert isinstance(res._value, pandas.Series)
+        assert res._value.equals(
+            pandas.Series([np.dtype(int), np.dtype(int)], index=["a", "a"])
+        )
+
+        # test that 'ModinDtypes.concat' with duplicated labels raises when not all dtypes are materialized
+        with pytest.raises(NotImplementedError):
+            res = ModinDtypes.concat(
+                [
+                    pandas.Series([np.dtype(int)], index=["a"]),
+                    DtypesDescriptor(cols_with_unknown_dtypes=["a"]),
+                ]
+            )
+
     def test_update_parent(self):
         """
         Test that updating parents in ``DtypesDescriptor`` also propagates to stored lazy categoricals.