TST: Add test case for groupby where by column contains values with same starting value

GH29635

Author: Mateusz

pandas/tests/groupby/test_groupby.py

@@ -2658,3 +2658,26 @@ def test_pad_backfill_deprecation():
         s.groupby(level=0).backfill()
     with tm.assert_produces_warning(FutureWarning, match="pad"):
         s.groupby(level=0).pad()
+
+
+def test_by_column_values_with_same_starting_value():
+    # GH29635
+    df = pd.DataFrame(
+        {
+            "Name": ["Thomas", "Thomas", "Thomas John"],
+            "Credit": [1200, 1300, 900],
+            "Mood": ["sad", "happy", "happy"],
+        }
+    )
+    aggregate_details = {"Mood": pd.Series.mode, "Credit": "sum"}
+
+    result = df.groupby(["Name"]).agg(aggregate_details)
+    expected_result = pd.DataFrame(
+        {
+            "Mood": [["happy", "sad"], "happy"],
+            "Credit": [2500, 900],
+            "Name": ["Thomas", "Thomas John"],
+        }
+    ).set_index("Name")
+
+    tm.assert_frame_equal(result, expected_result)