add assertion for columns not changed

Author: Genesis929

tests/system/small/test_dataframe.py

@@ -2956,6 +2956,9 @@ def test_join_different_table_with_duplicate_column_name(
     ].rename(columns={"int64_too": "int64_col"})
     pd_result = pd_df_a.join(pd_df_b, how=how, lsuffix="_l", rsuffix="_r")
 
+    # Ensure no inplace changes
+    pd.testing.assert_index_equal(bf_df_a.columns, pd_df_a.columns)
+    pd.testing.assert_index_equal(bf_df_b.columns, pd_df_b.columns)
     pd.testing.assert_frame_equal(bf_result, pd_result, check_index_type=False)