From what I've seen, doubloons in Pirate101 act the same as Wizard101's treasure cards. You can use them only once in battle. You can trade them with other pirates. However, you can sell or destroy unwanted treasure cards, while you can't do the same with doubloons.

It's been very annoying getting doubloons I don't want in battle, like those 'push enemies back' ones as it affects targeting a lot, those low level 'summon allies' if you're a higher level, and other ones that may not suit your class (strength doubloons show up all the time for me, a Musketeer who uses the agility attribute).

I'm just hoping that we can destroy doubloons in the future.

Egg timers!

There is a way:

(1) Create a new pirate (you may need to create a new account if your slots are currently full).
(2) Meet your new pirate somewhere
(3) Trade your unloved doubloons to the new pirate
(4) Delete the new pirate

-Indigo