So, recently I was looking at an answer, I clicked on the vote up button because it was good and then noticed I already had. The score changed to 2 and I un vote-upped it. Is this supposed to happen? The answer is here
Remember that you aren't the only one voting on questions and answers, more people do, actually, voting is the way to show your appreciation.
It can happen that if you upvote, somebody else might have done that too since you loaded the page. So after voting, the counter would have increased by 2 as if you did that, but in fact it is just more people voting while your page hasn't been refreshed yet. The system won't allow you to vote multiple times.
Please remember to vote often! That is much appreciated!