Diminishing returns may be a solution ...
That's how I tried to describe the idea:
"How about if after each vote on a specific account (including one's own account) each further vote on the same account would lead to significantly less curation reward for the voter and less profit for the upvoted account? Thus, when upvoting an account which I had already upvoted before, my voting power would be smaller than in case I upvote an account which I didn't upvote before."