I've had 2 Contracted Ubermensch (a fugitive one and a bonus one from researching the technology). Still, I could not hire any more for several turns (the (+) buton next to 'Contracted Ubermensch' was missing).

The problem went away after I exited and reloaded the game.

That's super weird, thanks for letting me know.

Fixed: I forgot to make the code check whether to show the "hire" button when you unlock a worker type that's already listed.