Improved error msg when a symbolic group name is redefined. Added docs

and NEWS.  Bugfix candidate?  That's a dilemma for Anthony <wink>:  /F
did fix a longstanding bug here, but the fix can cause code to raise an
exception that previously worked by accident.

Lib/sre_parse.py

@@ -81,8 +81,10 @@ class Pattern:
         gid = self.groups
         self.groups = gid + 1
         if name:
-            if self.groupdict.has_key(name):
-                raise error, "can only use each group name once"
+            ogid = self.groupdict.get(name, None)
+            if ogid is not None:
+                raise error, ("redefinition of group name %s as group %d; " +
+                              "was group %d") % (`name`, gid,  ogid)
             self.groupdict[name] = gid
         self.open.append(gid)
         return gid