So this suggestion is spurred from a mistake I just made (as some of the good ones are).

I bought a Research File Cabinet and planned to read it to my pet. When I clicked on the item, I accidentally chose to Open it instead of reading it to my pet. When I clicked Open, I was taken to a blank white page - no error, no notification. I went back to my inventory and the Research File Cabinet was gone. I had already unlocked the Mad Scientist last month, so this item tried to unlock an opponent I already had. Instead of giving me an error message saying "You have already unlocked this opponent.", it consumed the item.

I think it'd be a great idea to implement some type of an account check (similar to if a collection item is already in a collection). That way, if you have already unlocked the opponent, you will receive an error and your item will NOT be used. Then, you can sell the item or do whatever as you see fit.

This seems like a really big oversight; that definitely should not happen. Items with a purpose (such as foods/toys) don't disappear if their purpose is already fulfilled (when the pet's full/at max happiness) so items that unlock challengers shouldn't either.

And I'd say staff should give you another copy; you had no reason to believe it would do anything other than give you an error saying you already had the challenger.

I have already filed a ticket to see what can be done.

I'm hoping that this can get corrected - I'd hate to think how many people this may have happened to before.

Edit: My ticket has been resolved and I was told specifically "We do reimburse the items that unlock battle opponents because they're not supposed to be usable in that way again". Hopefully this means a fix can be put in place to produce an error.