Thinking of the Winter feast group mechanic, I have a suggestion for a mechanic that could double boss items but also take effort to fight:
A daruma doll
On the stomach of the daruma doll, you put the image of the boss you want to fight. Once the boss is inserted, one of it's eyes is crossed out.
When you fight the boss and win, the eyes are filled out and the doll becomes the duplicated item.
That way the doll isn't permanent and help duplicate for friends who want to fight with friends.