If you have the mount on another character it seems you can't by it again with GS-tokens. It won't show again. You can only buy via collection unlock after you first bought it.
If this is a bug or intended, I don't know.
Will be fixed (hopefully) with todays patch
Players can now have multiple copies of "Brrazz's Gift: The Master's Top Secret Army" in their inventory and bank.
German player here, yesterday me and my guildie talked about it, that it's still not fixed. Shouldn't be too hard I guess (but don't know for sure). Devs, please fix this minor bug.
@OwenBrooks is a really helpful community member. He's really tech savy and could have given you some helpful hints. He's no dev but I think have some connection to the EA-helpcenter. I'm quite sure he's not fishing.