Need help in making a debuff spell in CK

[OmenSakaw]

I've been trying to make a spell that when hit, would decrease enemy's magic resist for a few sec, just like one of the poisons.
It's all fine and working, but then I realized the effect would stack if I cast it multiple times on the same enemy, making it stupidly OP.

Is there a way to make it not stack, only reset the duration, when recast on the same enemy? Similar to buffs like Courage, which the health and stamina buff won't stack with multiple cast, only the duration.
I tried comparing both spells, but can't seem to find the option for this.

Any help would be appreciated!