TST: Add test for concat multiindex with category (#54103)

Add test for concat multiindex with category

Author: lukemanley

pandas/tests/reshape/concat/test_concat.py

@@ -832,3 +832,32 @@ def test_concat_mismatched_keys_length():
         concat((x for x in sers), keys=(y for y in keys), axis=1)
     with tm.assert_produces_warning(FutureWarning, match=msg):
         concat((x for x in sers), keys=(y for y in keys), axis=0)
+
+
+def test_concat_multiindex_with_category():
+    df1 = DataFrame(
+        {
+            "c1": Series(list("abc"), dtype="category"),
+            "c2": Series(list("eee"), dtype="category"),
+            "i2": Series([1, 2, 3]),
+        }
+    )
+    df1 = df1.set_index(["c1", "c2"])
+    df2 = DataFrame(
+        {
+            "c1": Series(list("abc"), dtype="category"),
+            "c2": Series(list("eee"), dtype="category"),
+            "i2": Series([4, 5, 6]),
+        }
+    )
+    df2 = df2.set_index(["c1", "c2"])
+    result = concat([df1, df2])
+    expected = DataFrame(
+        {
+            "c1": Series(list("abcabc"), dtype="category"),
+            "c2": Series(list("eeeeee"), dtype="category"),
+            "i2": Series([1, 2, 3, 4, 5, 6]),
+        }
+    )
+    expected = expected.set_index(["c1", "c2"])
+    tm.assert_frame_equal(result, expected)