Hey Bry,
I'm in the same boat. I try to be fair when I get the same lead from multiple sources.
First, I only send in one audition. I would never suggest sending the same audition thru multiple paths. That's a recipe for disaster!
As for who I choose, that will depend on a number of factors. If one audition notice comes in much earlier than the rest, I'll go with that one. If they all come in about the same time, I'll select based on my history with each agent, how often I've auditioned for them, etc.
I don't usually tell my agent that I've auditioned thru another source unless they ask. Even then, I will just say "another source", which may well include an online service.
Haven't had any issues with this approach yet. I'm pretty sure that most agents understand we have many audition sources.
One caveat: Watch out for exclusivity! If you have an agent that you've signed an exclusive contract for a region, honor that contract above other agencies.
I'm waiting for the exclusive-exclusive contract!
My 1/50th of a dollar,
Joe J Thomas
Joe J. Thomas Acting Portfolio