hey, i figured out a way to get 2 times insect swarm on one target.
1. cast highest rank of swarm (rank7)
2. then cast a lower rank (rank6)
the target will have the debuff from both insect swarms, but he ll only get damage for the swarm with the highest rank on the target. so maybe some ppl ll think this trick isnt useful, but you can use the swarms as a dispell protection for your other dots and debuffs.