Inconsistent handling of duplicate axes + mixed dtypes in DataFrame.w…

…here pandas-dev#25399 (pandas-dev#46139)

pandas/tests/frame/indexing/test_where.py

@@ -12,6 +12,7 @@
 from pandas import (
     DataFrame,
     DatetimeIndex,
+    Index,
     Series,
     StringDtype,
     Timestamp,
@@ -863,6 +864,21 @@ def test_where_none_nan_coerce():
     tm.assert_frame_equal(result, expected)
 
 
+def test_where_duplicate_axes_mixed_dtypes():
+    # GH 25399, verify manually masking is not affected anymore by dtype of column for
+    # duplicate axes.
+    result = DataFrame(data=[[0, np.nan]], columns=Index(["A", "A"]))
+    index, columns = result.axes
+    mask = DataFrame(data=[[True, True]], columns=columns, index=index)
+    a = result.astype(object).where(mask)
+    b = result.astype("f8").where(mask)
+    c = result.T.where(mask.T).T
+    d = result.where(mask)  # used to fail with "cannot reindex from a duplicate axis"
+    tm.assert_frame_equal(a.astype("f8"), b.astype("f8"))
+    tm.assert_frame_equal(b.astype("f8"), c.astype("f8"))
+    tm.assert_frame_equal(c.astype("f8"), d.astype("f8"))
+
+
 def test_where_non_keyword_deprecation(frame_or_series):
     # GH 41485
     obj = frame_or_series(range(5))