Hey Ebola

This is easy.

channeling:<spell> - Evaluates true if you are channeling the spell listed. With no spell listed, it evaluates true if any spell is being channeled.

You can modify it with no in front:

/cast [noChanneling:Hellfire] Hellfire

Spam that all day long and you will never double cast it until it stops casting or is interrupted

Good luck with the 5 locks and this strategy though I HIGHLY suggest a priest with PoH - CoH might be overkill for instances.