Playing around with the example Loranus gave, here's how I always saw this scenario working out (and I was pretty shocked when I first heard someone tell me it did double ID wounds)
If you have a Unit of Scarabs Toughness 3 with 3 wounds and an Armour save of 5+.
Good here.
I hit them with a Strength 6 AP - Flame Template manage to hit 4 of them. I roll and manage to wound with all 4. You now make armour saves. You made 1 so 3 unsaved wounds go into the wound pool
Good so far.
hich are now doubled because they were unsaved templates for swarms. Then you apply the first wound to the closest model which would be an instant kill. You then have 5 more wounds to apply since once they are unsaved they are doubled and put into the wound pool before going on models.
This is where I think you make a mistake on the process.
We have 3 wounds in the wound pool caused from a template. However, the wounds aren't smart. They don't know they're going to a unit with the swarm USR while they're sitting in the wound pool.
When the wound gets allocated to the scarab, you would double it per swarm, and then remove the base since the wound caused ID.
There are issues:
-what happens if a scarab has only 1 wound left? You can't keep allocating wounds to it, I'd say the wounds are lost. The base that took double wounds has been removed. If you bump the wound to another swarm, are they doubled against since it was a 2nd wound caused by a blast template? That seems silly to me.
-what happens if an IC (or other non-swarm) is in the unit? If you rolled to wound the closest model one at a time, double the wounds the swarm took, and removed it, you'd be resolving the hits the way I just described above.
GW just doesn't give us good guidance on exactly when to double the wounds a swarm takes. I'll play with it either way, but since there are multiple ways to look at the issue, I just wanted to know ahead of time how it would be played in BG.
It would make a lot more sense to me if you said "Well, 4 scarab bases are covered, so I roll 8 dice to wound them" or "4 scarabs and an IC are covered, so I roll 9 dice to wound them." Boom, solves the whole mess. Unfortunately, it's not written like that. :/