I have not deeply considered the first solution yet, but since the second solution also came across my mind, I have thought of some possible cases.
You are right. The duration of this spell is 3 seconds and according to
COMBAT_LOG_EVENT_UNFILTERED, this ticks every 0.5ish second and in total does 6 ticks.
BUT, I ain't sure if this is affected by haste...
1. Poison Bomb A (pbA) and Poison Bomb B (pbB)
Let's say pbA did spawn and already did some damage ticks. But, what if pbB newly spawns and does its first tick damage on the same time as pbA's next tick?
I know the chance is SUPER low, but theoretically it is possible, isn't it?
2. Poison Bomb A (pbA) + Enemy A (eA) and Enemy B (eB)
There's only one poison bomb, pbA, spawned underneath eA which already did couple of damage ticks, then eB suddenly jumps onto pbA. If poison bomb ticks same for both creature no matter whether they jumped on late or not, that should be fine. However, what if that single poison bomb ticks differently for each creatures?
This could be confirmed by testing, but I currently can't enter the game :/
EDIT #1 Thank you for warcraftlogs link!
I'll have a look into it
EDIT #2 I guess your first solution would be more suitable in this case.
It's more reasonable, straight-forward and just makes more sense. But, would still need more consideration to identify whether it's new cast or not.
According to the log, poison bombs are mostly spawned within 0.6 second after Rupture or Envenom, but there is still a case where it's spawned only 0.062 second after Rupture
OR where the current poison bomb's next tick is still done within 0.6 second after Rupture or Envenom.