Rogue Agent: Book #3
When the members of Witches Incorporated receive a visitor from an alternate reality, they are shocked to discover that Gerald Dunwoody is responsible for the chaos in this parallel world. This evil Gerald has turned into a raving tyrant whose use of black magic has made him unstoppable.
Unfortunately for them, this problem is about to cross over to their world very quickly because evil Gerald is searching for new worlds to conquer. And the only person who can stop him is good Gerald...who has just disappeared on assignment overseas.
Once they realize that Gerald has been kidnapped by his alternate self, Melissande and the rest of Witches Incorporated cross through to the alternate reality to save him.
But it appears that they're too late, because good Gerald has succumbed...and now there are two evil Geralds to contend with.