read_excel() modifies provided types dict when accessing file with duplicate column

pandas/io/parsers/python_parser.py

@@ -81,7 +81,10 @@ def __init__(self, f: FilePathOrBuffer | list, **kwds):
         self.verbose = kwds["verbose"]
         self.converters = kwds["converters"]
 
-        self.dtype = kwds["dtype"]
+        if isinstance(kwds["dtype"], dict):
+            self.dtype = kwds["dtype"].copy()
+        else:
+            self.dtype = kwds["dtype"]
         self.thousands = kwds["thousands"]
         self.decimal = kwds["decimal"]
 

pandas/tests/io/excel/test_readers.py

@@ -1278,6 +1278,24 @@ def test_ignore_chartsheets_by_int(self, request, read_ext):
         ):
             pd.read_excel("chartsheet" + read_ext, sheet_name=1)
 
+    def test_dtype_dict_unchanged_with_duplicate_columns(self, read_ext):
+        # GH 42462
+
+        filename = "test_common_headers" + read_ext
+        dtype_dict = {"a": str, "b": str, "c": str}
+        dtype_dict_copy = dtype_dict.copy()
+        result = pd.read_excel(filename, dtype=dtype_dict)
+        expected = DataFrame(
+            {
+                "a": ["1", "2", "3"],
+                "a.1": ["1", "2", "3"],
+                "b": ["b1", "b2", "b3"],
+                "c": ["c1", "c2", "c3"],
+            }
+        )
+        assert dtype_dict == dtype_dict_copy, "dtype dict changed"
+        tm.assert_frame_equal(result, expected)
+
 
 class TestExcelFileRead:
     @pytest.fixture(autouse=True)