"Rafael, ¿vienes mañana?"
Translation:Rafael, are you coming tomorrow?
Why not? Rafael, viene manana? Perhaps I don't know Rafael that well and the "usted " conjugation would be appropriate.
I'm not sure but perhaps the comma after his name is there purposedly for the reason that it's meant to address Rafael directly.
You could be asking someone else if Rafael is coming tomorrow, in which case wouldn't it be "viene"?
No indication of relationship therefore the question is yet another misleading piece of Duolingo .
There is no way that vienes is the only correct answer. The context does not reveal the relationship.