Resist the urge to repeat yourself.
This little snippet of advice applies to many areas of writing - words, scenes, plot elements.
But it also applies to characters, particularly secondary characters.
Each and every character in your story should have his own role and function.
Ask yourself if there is any one character who "repeats" another character - that is, serves the same function within the story.
Ask yourself: If I take this character out of this story, what will change? Will the story be affected? Will anybody miss him?
If not, it's probably time to murder your darlings.