fixed broken state after faulty columns assignment

dios/dios.py

@@ -198,9 +198,10 @@ class DictOfSeries:
 
     @columns.setter
     def columns(self, cols):
-        self._data.index = cols
-        if not self._data.index.is_unique:
+        index = pd.Index(cols)
+        if not index.is_unique:
             raise ValueError("columns index must have unique values")
+        self._data.index = index
 
     @property
     def values(self):