If the defending pokemon is already normally poisoned, could I "re-poison" the pokemon with crobat's Severe Poison attack? In other words, would the poison now inflict 40 damage between turns instead of the regular 10 even though the defending pokemon was already poisoned?
Yes. Whenever a Poison condition is applied to a Pokemon, it replaces the old condition.
So, in this case, the 4 damage counter Poison would replace the 1 damage counter poison.
Note that it can work the other way around, as well.
