Lol i love integers more than anything. I would just make an integer array called quest1 with an array of however many players there are, then set it to 1 when that player gets that quest. Then if he tries getting the quest again, it wont let him because the integer is 1, which means he already got it. Basically put your entire quest actions into a Then of If/Then/Else, where the If is
If quest1[player number of triggering player or whatever] is equal to 0, then
And then you put your quest, including to set quest1[player number of triggering player or whatever] equal to 1.
The else would be Do Nothing or display a game message that says "You cannot repeat the same quest twice" or something like that.