Sorry if there is actually a solution to this, I haven't been able to find one, but it's really hard sometimes to check for duplicates in the TC. For example, for my pet Glaskil I want one of every kind of fish/sea creature, but some of them look quite similar (jellyfish) and since there's no names on the items either it's a bit hard to tell sometimes.

Of course we shouldn't disallow duplicates completely because some people want to collect multiple of one item but a button to remove duplicates or something would be amazing.

A way to highlight duplicates would be really nice.

Maybe a button you could push while sorting your pet's treasure that highlights all the duplicates with a colored border, similar to we have for highlighting wishlist items.

Well they do this with food so it should be possible with TCs. When a pet has already eaten a food it says so. I would support this.

As long as it's optional, yes please.

The account search will tell you if you have more than one of an item in a tc. You could always do a search beforehand, I do it to make sure I don't already have a certain item and see if it's in a tc already.