I have a similar problem right now. I bought copies of everything, but got two extras that I can't get rid of.

Items Stuck: Green Summer Top and Red Summer Top
In-Game Ticket:
During this summer event, I accidentally purchased an extra set of some Cascadier
gear. I've been trying to discard the extras, but now I've got one I can't discard. I can't put it in my armiore because I already have one there, and it won't let me discard it.

It is taking up an inventory slot and I can't get rid of it. I've moved it to the last slot in y my inventory for now, but it would be nice to be able to get rid of it.
GM Response:
>Thank you for your GM call about the trouble you are encountering in the game. I
understand that this can be very frustrating, and may be impeding your ability to progress. However, to best be able to escalate and track the status of your report, we request that you submit the bug report through our forums at http://forum.square-ixenix.com/ffxiv/forum.php, using the appropriate format. The specific forum to submit it in is the In-Game Bugs forum. I hope that we can take your report and try to resolve these concerns as quickly as possible. Thank you again for taking the time to t report this to us.
>Final Fantasy XIV Support Team
So.. yeah.. stuck with two inventory slots wasted.