A decreasing voting impact for the same person could be a possible and simple solution to the problem.
It could be achieved in the same way as voting power decreases. I.e. if you vote for the same person your first vote is at 100% of your current voting power, your seconds is 90% and so on (with the decrease being debatable). Recharging again over time.
Sure, if you're voting on a specific pool of content mostly written by the same people there might be an issue. But with most people writing less than 1-3 posts per week, this should be fine.
Another issue might be comments in a conversation, where the value is added to each post. But considering that normally you'd upvote a comment with less than 100% voting power that shouldn't be an issue, as you could simply increase the voting power to your target level.
This proposal has two advantages:
- It's simple.
- At the same time, this would prevent the annoying self-upvoting of shitty one-line comments. (Brought here to the extreme, without content at all!!!)