Why not have two rogues? or ninjas? I dont see any problem of multiple "same classes" in group. It can be more of an joining thing, that dividing.
Group of rogues, forming an gang.. group of ninjas form clan... without taking anything away, instead adding to the mix. Character personality is the factor that makes the diffrence.
But if you think of addding something new to the mix, some divine caster might do wonders. Oracle for example (if feeling that religion is now your forte). Or pure fighter type. (I´we been reading the alternative character classes, and Hunter seems pretty intresting)