C++, skip adding already used old-style ids.

See michaeljones/breathe#266.

sphinx/domains/cpp.py

@@ -3759,8 +3759,14 @@ class CPPObject(ObjectDescription):
             else:
                 # print("[CPP] non-unique name:", name)
                 pass
-            for id in ids:
-                if id:  # is None when the element didn't exist in that version
+            # always add the newest id
+            assert newestId
+            signode['ids'].append(newestId)
+            # only add compatibility ids when there are no conflicts
+            for id in ids[1:]:
+                if not id:  # is None when the element didn't exist in that version
+                    continue
+                if id not in self.state.document.ids:
                     signode['ids'].append(id)
             signode['first'] = (not self.names)  # hmm, what is this abound?
             self.state.document.note_explicit_target(signode)