Front Crankshaft seal / belt issues

Ok I have a 09 Double cab Tacoma 4X4 with a 4.0 V6. Here's my problem at 185,000 miles I changed the sepertine belt and it came apart and pulled the front seal out. After changing the belt 3 times thanks to them being oil soaked found the leak and changed belt and front seal lat Saturday (5/9/15),on Monday the belt somehow slipped and it destroyed the belt and another front seal. Well as of today I have had to do this 2 more times and I have checked all the idler pulleys and tensioner and all spin identical and have same amount of play. Anyone else had this happen or maybe have a suggestion as to how to make this stop?