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How do you determine if they are using other accounts to vote on their own posts?

We don't, but that's something we're looking into. Step by step.

When two accounts both upvote each others comments within a minutes, over and over it can't be hard to detect.
See accounts ghazanfar.ali and leopard0505 comment history. Both are commenting on an article with a minute of each other and upvote each others comment, no way that is just automatic upvoting, it's clearly the same guy. That has to be a way to detect such obvious fake account upvoting

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