Many, many fish can tolerate conspecifics, but are you referring to a mated pair/trio or just having two of the same specie in a tank? Is this question referring to something you would like to house and gaining some ideas on?
All fish have their territorial disputes and/or hierarchies even within just a mated pair. I honestly cannot think of an animal that doesn't get along with a conspecific and, of course, depending on the environment including tank size.
Spotted Cardinalfish, Kaudern's Cardinal, Clown fish, Bartlett's Anthias, Firefish, etc. There are many fish that do well in pairs or groups. It all depends on the size of your tank and how much LR or hiding space you offer them.