Username already exist but It doesn't?

I want to change my username to "SmokingFlash" as it is my username in most gaming platforms and nobody has ever taken it from me. When I tried to change my username to this I get an error saying it already exists but I cant find the account. Id like to know if that username exists for an account and if it doesnt why I cannot change my name to my universal username (it is also the username of some of my linked accounts). It has never been taken before and so I'd just like to know if there is in fact an account with my username.

It is also possible this username is linked to one of my old emails so is there anyone I can contact with those emails to check if it is one of those associated with the name?