TST: Add test for duplicate by columns in groupby.agg using pd.namedAgg and asIndex=False (#58579)

* implement test for GH #58446

* Reformat GH issue comment

* Directly inline as_index=False in groupby call

---------

Co-authored-by: Jason Mok <[email protected]>

Author: jasonmokk

pandas/tests/groupby/test_groupby.py

@@ -2954,3 +2954,34 @@ def test_groupby_dropna_with_nunique_unique():
     )
 
     tm.assert_frame_equal(result, expected)
+
+
+def test_groupby_agg_namedagg_with_duplicate_columns():
+    # GH#58446
+    df = DataFrame(
+        {
+            "col1": [2, 1, 1, 0, 2, 0],
+            "col2": [4, 5, 36, 7, 4, 5],
+            "col3": [3.1, 8.0, 12, 10, 4, 1.1],
+            "col4": [17, 3, 16, 15, 5, 6],
+            "col5": [-1, 3, -1, 3, -2, -1],
+        }
+    )
+
+    result = df.groupby(by=["col1", "col1", "col2"], as_index=False).agg(
+        new_col=pd.NamedAgg(column="col1", aggfunc="min"),
+        new_col1=pd.NamedAgg(column="col1", aggfunc="max"),
+        new_col2=pd.NamedAgg(column="col2", aggfunc="count"),
+    )
+
+    expected = DataFrame(
+        {
+            "col1": [0, 0, 1, 1, 2],
+            "col2": [5, 7, 5, 36, 4],
+            "new_col": [0, 0, 1, 1, 2],
+            "new_col1": [0, 0, 1, 1, 2],
+            "new_col2": [1, 1, 1, 1, 2],
+        }
+    )
+
+    tm.assert_frame_equal(result, expected)