I recently tried to purchase 15 Curious Creatures Adoption Licenses at the Gem Store. After getting the Gems, I thought I purchased it... Turns out I bought the Desert ones, which I already unlocked all of them. Now I have 15 of these things in my storage... That's $70 down the drain.
I submitted a ticket to ANET to see if I can exchange it. Waiting for their response.
EDIT: Looks like, there's no way to really even warn you if you already have at least a mount from that specific License Type.