Can we add a little line in the item hover box to tell you how many of an item you already own? it would make it easier to stop double buying items you only need one of without having to do a search through your account every time for every item.

Like this?

No support. I don't really think this is necessary with the account search link already right there in the hover.

It also seems like something that would cause a ton of lag on pages with lots of items to load. A similar feature showing whether or not you owned items on the page was already removed from wishlists at one point due to how hard it was on the servers

I find it easier to just WL items in holiday shops and remove them from the wishlist as needed. An automatic account search for everything displayed on a page (or even every item hovered over) would be so much server strain and bog down the site so bad.

While I totally get where you're coming from and love the idea myself, others are right in saying it would be too much strain on the servers. Every item loaded on the page would need to pull from extra information and would most likely slow things down immensely. There are definitely other things that would need improvement before implementing something like this. The extra click for account search will have to do!