From 802a1dcdd1a4261bc0d4be62bbeeaae18e7286b5 Mon Sep 17 00:00:00 2001
From: Patrick Hoefler <61934744+phofl@users.noreply.github.com>
Date: Wed, 16 Jun 2021 03:46:04 +0200
Subject: [PATCH] Backport PR #42030: Regression raising Error when having dup
 cols with single dtype for read csv

---
 pandas/_libs/parsers.pyx                           | 2 ++
 pandas/io/parsers/python_parser.py                 | 2 ++
 pandas/tests/io/parser/dtypes/test_dtypes_basic.py | 9 +++++++++
 3 files changed, 13 insertions(+)

diff --git a/pandas/_libs/parsers.pyx b/pandas/_libs/parsers.pyx
index 7d7074988e5f0..e5e61e409c320 100644
--- a/pandas/_libs/parsers.pyx
+++ b/pandas/_libs/parsers.pyx
@@ -108,6 +108,7 @@ from pandas.core.dtypes.common import (
     is_object_dtype,
 )
 from pandas.core.dtypes.dtypes import CategoricalDtype
+from pandas.core.dtypes.inference import is_dict_like
 
 cdef:
     float64_t INF = <float64_t>np.inf
@@ -689,6 +690,7 @@ cdef class TextReader:
                                 count = counts.get(name, 0)
                             if (
                                 self.dtype is not None
+                                and is_dict_like(self.dtype)
                                 and self.dtype.get(old_name) is not None
                                 and self.dtype.get(name) is None
                             ):
diff --git a/pandas/io/parsers/python_parser.py b/pandas/io/parsers/python_parser.py
index 670868c6f4261..af25a4166d5a6 100644
--- a/pandas/io/parsers/python_parser.py
+++ b/pandas/io/parsers/python_parser.py
@@ -25,6 +25,7 @@
 )
 
 from pandas.core.dtypes.common import is_integer
+from pandas.core.dtypes.inference import is_dict_like
 
 from pandas.io.parsers.base_parser import (
     ParserBase,
@@ -424,6 +425,7 @@ def _infer_columns(self):
                                 cur_count = counts[col]
                             if (
                                 self.dtype is not None
+                                and is_dict_like(self.dtype)
                                 and self.dtype.get(old_col) is not None
                                 and self.dtype.get(col) is None
                             ):
diff --git a/pandas/tests/io/parser/dtypes/test_dtypes_basic.py b/pandas/tests/io/parser/dtypes/test_dtypes_basic.py
index 59fd3de60e0bf..bc20f1d1eea5f 100644
--- a/pandas/tests/io/parser/dtypes/test_dtypes_basic.py
+++ b/pandas/tests/io/parser/dtypes/test_dtypes_basic.py
@@ -248,3 +248,12 @@ def test_dtype_mangle_dup_cols(all_parsers, dtypes, exp_value):
     result = parser.read_csv(StringIO(data), dtype={"a": str, **dtypes})
     expected = DataFrame({"a": ["1"], "a.1": [exp_value]})
     tm.assert_frame_equal(result, expected)
+
+
+def test_dtype_mangle_dup_cols_single_dtype(all_parsers):
+    # GH#42022
+    parser = all_parsers
+    data = """a,a\n1,1"""
+    result = parser.read_csv(StringIO(data), dtype=str)
+    expected = DataFrame({"a": ["1"], "a.1": ["1"]})
+    tm.assert_frame_equal(result, expected)