Fix mypy and improve what's new

Author: luke

doc/source/whatsnew/v2.0.0.rst

@@ -1366,7 +1366,7 @@ Reshaping
 - Bug in :meth:`DataFrame.explode` raising ``ValueError`` on multiple columns with ``NaN`` values or empty lists (:issue:`46084`)
 - Bug in :meth:`DataFrame.transpose` with ``IntervalDtype`` column with ``timedelta64[ns]`` endpoints (:issue:`44917`)
 - Bug in :meth:`DataFrame.agg` and :meth:`Series.agg` would ignore arguments when passed a list of functions (:issue:`50863`)
-- Bug in :meth:`DataFrame.agg` and :meth:`Series.agg` would return incorrect type when dist-like argument passed in (:issue:`51099`)
+- Bug in :meth:`DataFrame.agg` and :meth:`Series.agg` on non-unique columns would return incorrect type when dist-like argument passed in (:issue:`51099`)
 
 Sparse
 ^^^^^^

pandas/core/apply.py

@@ -411,7 +411,7 @@ def agg_dict_like(self) -> DataFrame | Series:
             context_manager = nullcontext()
 
         if isinstance(selected_obj, ABCDataFrame):
-            is_non_unique_col = selected_obj.columns.duplicated()
+            is_non_unique_col = selected_obj.columns.duplicated().tolist()
         else:
             is_non_unique_col = [False]