[Mod Question]How to make a spell dispel itself on recasting?

[EMPTYd]

I'm gonna make a spell with really really long duration, and I wish it to be able to "switch on/off".

 

Basicly I want it to take effect by casting once, and dispel by casting again.

 

Is this possible? What script should I write? I wrote one to monitor the spell effect and dispel it if player already has one, but it doesn't work.