How often do you read separate books by the same author and find yourself thinking, "This guy seems familiar," and then realizing that you remember him from a different book by the same author--as a different character?
I think writers have a tendency to do that because we write what they know. Sure, we can dream up characters (quirks, characteristics, etc.) by eventually, especially if these are main characters, they're going to be drawn from people the writers know, or at least readily supplied by his or her psyche. After all, characters -- as real as they may seem -- are only extensions of the author's imagination.
Especially if you have written 15, 20 novels, how can you ever avoid writing a similar character? After all, I don't think there are really that many fundamentally different types of people in real life anyway. Sure, we're all different with different backgrounds, etc. but in many ways, a lot of us are very similar in hopes, dreams, temperaments, personalities, behaviors, etc. We can tweaked the quirks and ticks, but basically, people are rather alike.
In my own writing, I try different things, and the characters who speak to me seem to be very different. Greg Lockland in The Pacific Between is very different than Kai in my WIP, but they still share similar traits (partially because they're both the "heroes"). My female leads are rather different, partially because one is still very young (15 years old).
Personally I don't make a conscientious effort of making my characters all different. I don't draw character charts and say, "Hey, I already have someone like this, so I should make him this way so that he's different..." I just let my characters speak to me and show me what they're like and what they'd do. I understand there would be limitations, but at least these characters feel organic an real to me instead of some puppets or quirky characters for quirkiness' sake.