I use animation event,which should be called once when the animation loop once.the trouble is：There is a small possibility(2 or 3 in every 100 times) that this event function is called twice…is this a unity bug???
Just try to avoid putting the even on the last frame , sometimes it acts strange;
as i can remember, it was ignored sometimes 
So, try to put it 2 or 3 frames early it will not affect the result i guess ( make sure the animation not PingPong, or it will truly be twice);
which Unity version you use ?
its not the last frame,and not pingpong.I am sure…its in the middle of the animation…I use version 3.4