Fix left join turning into outer join

pandas/core/frame.py

@@ -5328,18 +5328,17 @@ def _join_compat(self, other, on=None, how='left', lsuffix='', rsuffix='',
                 raise ValueError('Joining multiple DataFrames only supported'
                                  ' for joining on index')
 
-            # join indexes only using concat
-            if how == 'left':
-                how = 'outer'
-                join_axes = [self.index]
-            else:
-                join_axes = None
-
             frames = [self] + list(other)
 
             can_concat = all(df.index.is_unique for df in frames)
 
+            # join indexes only using concat
             if can_concat:
+                if how == 'left':
+                    how = 'outer'
+                    join_axes = [self.index]
+                else:
+                    join_axes = None
                 return concat(frames, axis=1, join=how, join_axes=join_axes,
                               verify_integrity=True)
 

pandas/tests/frame/test_join.py

@@ -31,6 +31,11 @@ def right():
     return DataFrame({'b': [300, 100, 200]}, index=[3, 1, 2])
 
 
+@pytest.fixture
+def right_non_unique():
+    return DataFrame({'c': [400, 500, 600]}, index=[2, 2, 4])
+
+
 @pytest.mark.parametrize(
     "how, sort, expected",
     [('inner', False, DataFrame({'a': [20, 10],
@@ -165,3 +170,11 @@ def test_join_period_index(frame_with_period_index):
         index=frame_with_period_index.index)
 
     tm.assert_frame_equal(joined, expected)
+
+
+def test_join_left_sequence_non_unique_index(left, right, right_non_unique):
+    # left join sequence of dataframes with non-unique indices (issue #19607)
+    joined = left.join([right_non_unique], how='left')
+    tm.assert_index_equal(
+        joined.index.unique().sort_values(),
+        left.index.sort_values())