The thing about metaphors is that the become such a part of the language that we cease to realise what they mean. This has happened, I think, to our expression “a one-off”, as in “There’s no one like Bill Bloggs, he’s a real one-off”.
This comes from the world of mass production, where everything that comes off the conveyor belt is identical. Any “tailor-made” (another image) or more accurately “custom-made” item can also be referred to as a “one-off”.
Now you want to know what that is in Spanish – it is “fuera de serie”. The “serie” or series is the line of production, and something “fuera de serie” stands out from it as different.
In Spanish a person who is “fuera de serie” can be just different – an odd-ball (where does that comes from?!) or also outstanding. – you know like Messi or Nadal.