Erm, no one HAD to be targeted twice. It's easier if you make it like a chain of names because it means you can have an odd or even amount of players , for example:
Hasit > Dumbledore > Voldemort > Harry Potter > Hasit
so Hasit is targetting Dumbledore, Dumbledore is targeting Voldemort ,Voldemort is targetting Harry Potter and Harry Potter is targeting Hasit.
Then If someone dies or quits you simply take them out of the chain of names. so if Hasit was to die then the chain would look like this:
Dumbledore > Voldemort > Harry Potter > Dumbledore
and now Harry Potter is targetting Hasits target which was Dumbledore.
I think that was how the other Game Hosts sorted out who targets who in the previous Forum Assassination games, Only problem was it made the game predictable and player were able to work out exactly who was targeting who which isn't much of a problem but it annoys some people.
So in some ways your way was better, because it made it impossible to actually work out who was targeting who without asking the people themselves which I think made the game more interesting but it messed it up a little as well.
