This works only on other people question. If You click on upvote button very fast twice it will add 2 upvotes most of the times. If You click then twice fast You can remove 2 points to original amount. Real problem starts when You click once to remove point and then refresh page. Now You can add points again, so by adding 2 points and removing 1 You can probably add infinite amount of points to some post.
Upvote persist when opened in another browser or computer.
You can also use this to superdownvote someone post.
I found out and tested using this question (How can i spawn actors based on an image mask? - Blueprint - Unreal Engine Forums) There are 5 upvotes, all by me. (I came to higher values).
Those extra upvotes doesn’t seem to add up to user score, as SarahLee1 score did not increase more than up to 6 points.