So, I'm going to be completely honest and say I don't like voting! its just simply not something i'm fond of doing....but with the GREAT new rewards for doing it, y'all leave me no choice but to vote daily! Anyhow, My suggestion is adding a small(How hard could it be?) feature that simply grays-out the sites you already voted for, or moves them to a side-box so that you don't try to vote on the same site twice(Which today alone I did twice). Also, Adding a link to the wiki page about voting(where it tells you the rewards and such).
When you click on the browser tab for the vote page after you finished voting for a site, the last link you clicked to vote on is highlighted on the vote page, so you can keep track of which site you're on. Only problem with that, is as soon as you move your cursor, the highlight goes away. I use that method, along with just keeping a mental count of how many I've voted on.
That'll work, but its more convenient if the site itself keeps track... not a huge problem, but for us that are impatient/ADHD etc, who don't pay enough attention, just a little bit annoying to keep clicking the same one. I can't remember the site name(Not sure if i'd be allowed to post it if I did....it was a RSPS), but there was one site that each time you voted the site link "Link-name-here.com" would basically go into limbo, floating around the screen, just not infront of other links.... I liked that method haha
