Prevent duplicate entrance entries

BaseClasses.py

@@ -1843,7 +1843,8 @@ class Entrance(object):
         self.target = target
         self.addresses = addresses
         self.vanilla = vanilla
-        region.entrances.append(self)
+        if self not in region.entrances:
+            region.entrances.append(self)
 
     def __str__(self):
         return str(self.__unicode__())

Main.py

@@ -863,7 +863,9 @@ def copy_world_premature(world, player):
             for location in copied_region.locations:
                 location.parent_region = copied_region
             for entrance in region.entrances:
-                copied_region.entrances.append(ret.get_entrance(entrance.name, entrance.player))
+                ent = ret.get_entrance(entrance.name, entrance.player)
+                if ent not in copied_region.entrances:
+                    copied_region.entrances.append(ent)
             for exit in region.exits:
                 if exit.connected_region:
                     dest_region = ret.get_region(exit.connected_region.name, region.player)