Avoid duplication on ban voting

[d1msdale]

Sometime when you couple of times click for voting - your vote could be duplicated. Current logic doesn't check voter for unique. I added check for this. Also I have suspicious that should change checking from _id to id in all cases, because by mongo logic '_id' and 'id' are different fields an there could be potential collision

Like a proposition should change

from push to array to update

[backmeupplz]

hah, that would be nice

[d1msdale]

@backmeupplz I can't create PR. I suppose because I'm not in contributor list.

[backmeupplz]

@d1msdale you should be able to :) fork and then create a PR from your fork