The only two math projects that I am aware of doing double work was GIMPS vs Mersenne@home, the latter is closed (it was a waste of energy due to lack of knowledge by crunchers side), and probably now http://oproject.goldbach.pl/index.php vs http://www.ieeta.pt/~tos/goldbach.html. In contact with our fellow Tomás Oliveira he can't tell me for sure if there's double work on the goldbach conjecture because the former project doesn't publish very well the results. So he has lots of doubts.