I have been into my wishlist and there are 2 of almost everything and you cant the delete the 2nd item.

Im having a similar problem, when I look at the stickers In my WL, it shows a lot that I have actually bought, if I search for it via the icon it says I dont have it, but when I click to sort by the user price, they all disappear, I can make it happen over and over.
Have to tried sorting them, and does the duplicate disappear when you do?

I have removed some of the duplicates only to fine they might have come back later on.

have you tried clicking user price, do they disappear, but come back if you click a different option? mine do.

You might want to check out this Wishlist adding duplicates