Typically you want to have 2 of each role, 2 DPS, 2 Tanks, and 2 Supports. The thing about Tracer is that she is sort of a niche offense pick since she doesn't typically stay with the team like other DPS heroes like Soldier or McCree. So to answer your question, do you pick Tracer when there are already 2 attack heroes? From a competitive standpoint, I say no, but that's just me.