No repercussions. It would take too much processing power to keep everyone honest. The site already crashes fairly regularly.
Steem Engine is good for now, surprisingly, but the fact that they can't stop you from upvoting yourself will eventually kill the service. All it's going to take is like 20 or 30 dolphins (or even minnows) upvoting accounts only owned by them to completely undermine the upvote for upvote mechanic. Soon enough, everyone on the front page will only upvoting themselves instead of the Steem Engine community.
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well most people just have one steemit account though. I never upvoted myself on there. I always upvote other people's post that I find interesting. Also, I never unfollow anyone so there are plenty of people like me that use it