rewrite _merge_indexes

Author: araraonline

pandas/core/indexes/api.py

@@ -278,32 +278,34 @@ def _merge_index_list(index_list,
         duplicate values. This is error is not raised if
         allow_matching_dups=True and all the indexes have a common identity.
     """
-    if verify_dups or sort:
-        has_dups = any(ix.has_duplicates for ix in index_list)
-
-    if sort and has_dups:
-        raise _CannotSortDuplicatesError("Cannot sort an index that"
-                                        " contains duplicate values.")
-
-    if has_dups and not allow_matching_dups:
-        raise _DuplicatesError("Index has duplicate values.")
-
-    uindex_list = com.get_distinct_objs(index_list)  # unique index list
+    # unique index list (a is b)
+    uindex_list = com.get_distinct_objs(index_list)
+
+    # verify duplicates
+    if sort or verify_dups:
+        has_dups = any(ix.has_duplicates for ix in uindex_list)
+        if has_dups:
+            if sort:
+                raise _CannotSortDuplicatesError("Cannot sort an index that"
+                                                " contains duplicate values.")
+            elif verify_dups and not allow_matching_dups:
+                raise _DuplicatesError("Index has duplicate values.")
+            elif verify_dups and allow_matching_dups and len(uindex_list) >= 2:
+                raise _DuplicatesError("Index has duplicate values and does"
+                                      " not match other indexes.")
+
+    # edge results
     if len(uindex_list) == 0:
         return pd.Index()
     elif len(uindex_list) == 1:
         return uindex_list[0]
 
-    # index_list has at least two indexes
-    if has_dups:
-        raise _DuplicatesError("Index has duplicate values and does"
-                              " not match other indexes.")
-
-    # index_list has no indexes with duplicates
+    # reduce to one result
     result = uindex_list[0]
     for idx in uindex_list[1:]:
         result = _merge_indexes(result, idx)
 
+    # sort
     return result if not sort else _sort_index(result)