If a person creates sock puppets in order to agree with himself, that person must be an adept liar . . . or a meticulous record keeper with the sock puppet's bio at his fingertips.
But a sock puppet or two created solely to vote for one's own posts is relatively easy to accomplish. Strange and obsessive, but easy to do.
personally, I don't have the time for that sort of nonsense.
However, when someone asks me more than once a month if I voted his post(s) down, and, if, I have a day when I check on my own posts more than once and I see the votes go up and down and know that only two people have posted, I do suspect that the worried poster has alter egos that were created to make him look better than he is.