At first I thought this was just me, but then I've seen it on others' WLs too. Sometimes - not always - when I add an item to my wishlist and then go to Unsorted to look at it, the WL will have multiples of the same item. Sometimes two, sometimes three, sometimes four. The organising function treats them as separate, but I think the hover and "Add/Remove From Wishlist" button does not.

Wishlist adding duplicates