ensure get_index_level is called early in code

janitor/functions/coalesce.py

@@ -113,11 +113,13 @@ def coalesce(
     if default_value:
         check("default_value", default_value, [int, float, str])
 
-    if target_column_name is None:
-        target_column_name = column_names[0]
-
     outcome = df.loc(axis=1)[column_names].bfill(axis="columns").iloc[:, 0]
     if outcome.hasnans and (default_value is not None):
         outcome = outcome.fillna(default_value)
 
+    if target_column_name is None:
+        df = df.copy()
+        df.iloc[:, 0] = outcome
+        return df
+
     return df.assign(**{target_column_name: outcome})

tests/functions/test_coalesce.py

@@ -63,3 +63,22 @@ def test_coalesce_without_delete():
     expected = df.assign(s3=df.s1.combine_first(df.s2).fillna(0))
     result = df.coalesce("s1", "s2", target_column_name="s3", default_value=0)
     assert_frame_equal(result, expected)
+
+
+def test_coalesce_duplicate_columns():
+    """
+    Test output on duplicate columns.
+    """
+    df = pd.DataFrame(
+        np.array([[1.0, 2.0, 2.0], [np.nan, 3.0, np.nan], [3.0, 1.0, 9.0]]),
+        columns=["a", "a", "c"],
+    )
+
+    expected = pd.DataFrame(
+        np.array([[1.0, 2.0, 2.0], [3, 3.0, np.nan], [3.0, 1.0, 9.0]]),
+        columns=["a", "a", "c"],
+    )
+
+    actual = df.coalesce("a")
+
+    assert_frame_equal(expected, actual)