Fix refs appearing in dupes-only view

* Some refs appeared in the dupes-only view after a re-prioritization was done a second time.
* It seems the core.Results.__dupes list was not properly updated whenever core.app.Dupeguru.reprioritize_groups() -> core.Results.sort_dupes() was called.
When a re-prioritization is done, some refs became dupe, and some dupes became ref in their place. So we need to update the new state of the internal list of dupes kept by the Results object, instead of relying on the outdated cached one.
* Fix #757.

core/results.py

@@ -94,7 +94,6 @@ class Results(Markable):
 
     # ---Private
     def __get_dupe_list(self):
-        if self.__dupes is None:
         self.__dupes = flatten(group.dupes for group in self.groups)
         if None in self.__dupes:
             # This is debug logging to try to figure out #44