Is resistance Coup worth getting if I already have resistance avalon?
I'd like to post an additional, though not entirely different answer to Joey's. 

Coup is a completely different game. 

Key distinguishing elements are:


Coup has no co-operative element like The Resistance does, it's entirely out for yourself and trying to outlive all the other players. 
Coup is a very short game - games can be over in 30 seconds (for two people) to 10 minutes for a long game. 
Coup can be played by 2-6 people. 


In my opinion Coup is a very good addition to your board game collection. For its price and size, it's an easy game to add. 

Basically - Coup is a very good warm up game.  While everybody is getting settled, or you are waiting for people to arrive, you can play a few games of coup.  The rules are nice and simple, most people understand them after one short game. Because the games are so short, it doesn't matter if someone arrives half way through, because they can just join the next game.