TST: Add test for union with duplicates (#40967)

Author: phofl

pandas/tests/test_algos.py

@@ -2418,10 +2418,16 @@ def test_diff_low_precision_int(self, dtype):
         tm.assert_numpy_array_equal(result, expected)
 
 
-def test_union_with_duplicates():
+@pytest.mark.parametrize("op", [np.array, pd.array])
+def test_union_with_duplicates(op):
     # GH#36289
-    lvals = np.array([3, 1, 3, 4])
-    rvals = np.array([2, 3, 1, 1])
-    result = algos.union_with_duplicates(lvals, rvals)
-    expected = np.array([3, 3, 1, 1, 4, 2])
-    tm.assert_numpy_array_equal(result, expected)
+    lvals = op([3, 1, 3, 4])
+    rvals = op([2, 3, 1, 1])
+    expected = op([3, 3, 1, 1, 4, 2])
+    if isinstance(expected, np.ndarray):
+        result = algos.union_with_duplicates(lvals, rvals)
+        tm.assert_numpy_array_equal(result, expected)
+    else:
+        with tm.assert_produces_warning(RuntimeWarning):
+            result = algos.union_with_duplicates(lvals, rvals)
+        tm.assert_extension_array_equal(result, expected)