How do we change our FC Abbreviation?

Because somehow, we have the same Abbreviation as someone else's FC. (Ragnarok)

http://i.imgur.com/rDNepSD.jpg

I'm guessing from the lack of replies for nearly a year... that you can't do this :V

go to your Grand Company that the FC is pledge with, ask one of the NPC there about it.

I'm really surprised that FC tags can be duplicated. :/

Correct, talk with the free company administrator in the grand company office and you should be good to go. :)