your mechanic is incompetent. He should of done it right the first time. If he was really as experienced as he believed he was he would have anticipated where possible leaks would occur and taken measures to make sure it became resolved. Apparently your mechanic has never heard of Murphy`s Law...What ever can go wrong WILL...Its bad business for everyone to have to do thinkgs twice especially when it comes to tranny work. From the sound of it, he may have not properly sealed the mating surfaces around the areas of the rear main seal. The other thing that may have happened is the drive shaft seals could have been damaged during reassembly. Im not really sure about that though. Pulling a tranny is not that hard. But I would get a Helms manual before I started. Good luck