I was playing with Horatio and, for the purpose of the steam achievement, I selected for the game also an AI Horatio player. 

When I met him I thought: "Why should I be at peace with this impostor?"

The idea is then to add in the New Game screen an option that prevents to be at peace players using the same faction.

Maybe it could also be fun to add some different text in the diplomatic sceen when you meet another player with your same faction.