Light and Darkness Immuto

[Hellomurse]

"Any character that suffers damage from the spell also gains the Blind Condition until the start of the caster's next turn.
This Immuto may be added to a spell multiple times. Each time, increase the duration of the Blind Condition by +1 round."
 

Do multiple hits from a spell with these Immuto in a single round increase the duration, or does the duration increase only happen when these Immuto are applied multiple times in a single casting?

[diki]

I haven't read the rules in a while but I think it doesn't stack so you would only apply the longest duration of either the targets current condition or the new casting one.

Light isn't going to blind you that much if you are already blinded?

(I think it's a game balance thing to encourage you to stack multiple instances)