i never understood why so many fail simple looping in animation. upon repeat all you have to do is make sure the same frame isn't shown twice. otherwise you get 1 frame twice as long as the others and thus the loop doesn't appear to be seamless. it's basics.
in flash if you have a tween that goes back to the starting keyframe and then loops (very common) you either cut that tween 1 frame short by creating a new last keyframe that isn't identical to the start, or you use actionscript and gotoandplay to the starting keyframe on the ending keyframe (which ensures the ending keyframe isn't drawn)