I checked what will happen with duplicated items and it looks game handeled with them pretty well.
Extra healing stones dissapeared after completing a quest.
Additional teleportation pyramyde was concidered as the second one thus when I droped it and picked up I recived experience for quest and pyramide in catacombs hasn't been spawned.
Only thing that left is magic mirror. I guess dumping it somewhere and forgetting about it won't hurt the game.
