even if you dont change keyframe settings the chance of the same particles occuring again
are very slim.
just take a section thats say 12 seconds. take the last two seconds and add to the front. then mix this layer out from 100 to 0, and at the same time mix in the lower level 0 to 100 (do these at the same keyframes as each other) then using the velocity customize the mix with maybe one fading out quicker
than the other, believe me ive been using ae for 8 years and this is still the best kind of approach if
your making seamless looping sequences.