Update tests and exception message.

The exception is also raised for `Panel` but not for `Series` objects.

Fixes pandas-dev#7813.

Author: filmor

pandas/io/pytables.py

@@ -2701,9 +2701,11 @@ def write(self, obj, **kwargs):
 
         self.attrs.ndim = data.ndim
         for i, ax in enumerate(data.axes):
-            if i == 0:
-                if not ax.is_unique:
-                    raise ValueError("Columns index has to be unique for fixed format")
+            if i == 0 and not ax.is_unique:
+                raise ValueError(
+                        "Columns index (resp. Items axis for Panels) "
+                        "has to be unique for fixed format"
+                        )
             self.write_index('axis%d' % i, ax)
 
         # Supporting mixed-type DataFrame objects...nontrivial

pandas/io/tests/test_pytables.py

@@ -4571,6 +4571,35 @@ def test_duplicate_column_name(self):
             other = read_hdf(path, 'df')
             tm.assert_frame_equal(df, other)
 
+    def test_non_unique_items_axis(self):
+        panel = Panel(
+                    data=np.zeros((2, 2, 2)),
+                    items=["a", "a"],
+                    major_axis=["b", "b"],
+                    minor_axis=["c", "c"]
+                )
+
+        with ensure_clean_path(self.path) as path:
+            self.assertRaises(
+                    ValueError, panel.to_hdf, path, 'panel', format='fixed'
+                    )
+
+            panel.to_hdf(path, 'panel', format='table')
+            other = read_hdf(path, 'panel')
+            tm.assert_panel_equal(panel, other)
+
+    def test_non_unique_series_index(self):
+        series = Series([0, 0], index=["a", "a"])
+
+        with ensure_clean_path(self.path) as path:
+            series.to_hdf(path, 'series', format="fixed")
+            other = read_hdf(path, 'series')
+            tm.assert_series_equal(series, other)
+
+            series.to_hdf(path, 'series', format='table')
+            other = read_hdf(path, 'series')
+            tm.assert_series_equal(series, other)
+
 
 def _test_sort(obj):
     if isinstance(obj, DataFrame):