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I recently checked the daily reputation gain of some of the more well established users on SO simply out of curiosity. Then I noticed this

a user had lost 1100 reputation during a single day and when I looked closer

I found out that the whole reputation loss was made because of unupvotes at the exact same time (14:43).

First I thought that SO did some clean up and deleted some users which revoked upvotes and made this user lose the reputation. So I decided to look into some other users to see if they also experienced reputation loss on the same date and time but I have not found a single user that has any rep loss during this day. This made me suspect that it is only one user that has been deleted but then how could the reputation loss be so great for this user? I am not well established on SO but I find it unexpected that this single, now deleted, user then had upvoted this user on over 100 different answers.

This made me think, what does Stack Overflow do to prevent a person to create two different accounts on the website, get one of them to over 15 reputation, and then use one of the accounts to upvote the other account's answers and questions to gain some extra reputation? Is there any known algorithm on the site that prevents one user to boost another user's reputation gain like this? Surely I am not the only one that has thought of this scenario so I hope that there is some measures taken from SO to prevent this from happening but I have failed to find any information of this online.

I am not making any accusations that this user has boosted his reputation gain by making a secondary account. For all that I know the upvotes may be completely legit and come from different users. This kind of major reputation loss is perhaps more common on SO that I know and is not a surprising sight.

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Is there any cap to how much a user can upvote another user on SO?

There is no system level block at the moment of voting. That would be too easy to detect and circumvent.

This made me think, what does Stack Overflow do to prevent a person to create two different accounts on the website, get one of them to over 15 reputation, and then use one of the accounts to upvote the other account's answers and questions to gain some extra reputation? Is there any known algorithm on the site that prevents one user to boost another user's reputation gain like this?

They run a script each night to correct those so-called 'serial votes'. Sometimes, it doesn't work and if users detect such cases, they can report them to ♦ moderators who can invalidate serial voting as well.

I am not well established on SO but I find it unexpected that this single, now deleted, user then had upvoted this user on over 100 different answers.

Most likely with a number of corrections like this, there are actually several accounts involved.

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