REGR: Fix bug when replacing categorical value with self

doc/source/whatsnew/v1.1.0.rst

@@ -293,6 +293,7 @@ Categorical
 - Bug when passing categorical data to :class:`Index` constructor along with ``dtype=object`` incorrectly returning a :class:`CategoricalIndex` instead of object-dtype :class:`Index` (:issue:`32167`)
 - Bug where :class:`Categorical` comparison operator ``__ne__`` would incorrectly evaluate to ``False`` when either element was missing (:issue:`32276`)
 - :meth:`Categorical.fillna` now accepts :class:`Categorical` ``other`` argument (:issue:`32420`)
+- Bug where :meth:`Categorical.replace` would replace with ``NaN`` whenever the new value and replacement value were equal (:issue:`33288`)
 
 Datetimelike
 ^^^^^^^^^^^^

pandas/core/arrays/categorical.py

@@ -2463,7 +2463,8 @@ def replace(self, to_replace, value, inplace: bool = False):
                 if new_value in cat.categories:
                     value_index = categories.index(new_value)
                     cat._codes[cat._codes == index] = value_index
-                    cat.remove_categories(replace_value, inplace=True)
+                    if new_value != replace_value:
+                        cat.remove_categories(replace_value, inplace=True)
                 else:
                     categories[index] = new_value
                     cat.rename_categories(categories, inplace=True)

pandas/tests/arrays/categorical/test_algos.py

@@ -64,6 +64,7 @@ def test_isin_cats():
     [
         ("b", "c", ["a", "c"], "Categorical.categories are different"),
         ("c", "d", ["a", "b"], None),
+        ("a", "a", ["a", "b"], None),
         ("b", None, ["a", None], "Categorical.categories length are different"),
     ],
 )