TST: Add test for mangling of unnamed columns

xref gh-13017.

Author: gfyoung

pandas/tests/io/parser/mangle_dupes.py

@@ -86,3 +86,19 @@ def test_thorough_mangle_names(self):
                                mangle_dupe_cols=True)
             assert list(df.columns) == ["a", "a.1", "a.3", "a.1.1",
                                         "a.2", "a.2.1", "a.3.1"]
+
+    def test_mangled_unnamed_placeholders(self):
+        # xref gh-13017
+        orig_data = {"0": [1, 2, 3]}
+        df = DataFrame(orig_data)
+
+        # This test recursively updates `df`.
+        for i in range(3):
+            exp_data = {"Unnamed: 0" + ".1" * j: [0, 1, 2]
+                        for j in range(i + 1)}
+            exp_data.update(orig_data)
+
+            expected = DataFrame(exp_data)
+            df = self.read_csv(StringIO(df.to_csv()))
+
+            tm.assert_frame_equal(df, expected)