So my Wind-up Bahamut code has already been used...?

[Cigaran]

Also did add your code with all the letter done in CAPS?

This. Most companies don't have their codes set up to be case sensitive but Square Enix seems to have it as a requirement. I got the same error with mine when I entered it today. Cleared it out and input the code again with Cap Locks enabled and the code was accepted without issue.