This is insightful especially since we have thought about this problem and had not come up with a solution. I am assuming all the icons will be unique though, else it may lead to a lot of confusion. Also, I believe people choose icons that they best identify with and if it is not unique their association with the icon may not be as strong.
Another thing I have experimented with is if you can upload a picture and create a hand drawn painting / image like Chris Dixon's or Fred Wilson's image which I think are brilliant in creating a brand and identity.
I am assuming all the icons will be unique though, else it may lead to a lot of confusion
I think this might just be a programmer's way of (over)thinking it : "the users are unique, so the avatars must be unique, or chaos ensues". In actuality it will just be mildly confusing at best, but nothing earth shattering. You still have the name next to the avatar for reference, as well as the project, and with several dozens of them, most will be different still.
Another thing I have experimented with is if you can upload a picture and create a hand drawn painting / image like Chris Dixon's or Fred Wilson's image which I think are brilliant in creating a brand and identity.