I dont know the odds, but i have heard that it would be very unlikely. surely if you did meet someone and suspect it then you could find out from the clinic, they would have records of the donors and although they couldnt tell you who it was, maybe they could tell you if its the same one. sorry not to be that much help
What are the odds? Probably greater than you would think.
The Journal of Pediatrics had an article about a single donor in Michigan who had passed on a genetic defect. His sperm had been used 11 times, and 5 of those children had the exact same pediatrician.
Have you tried signing up at a donor sibling registry?