re: "How do I get someone to back out a bonus? I paid a bonus, but mistakenly paid twice."
There is no button for "undoing" a bonus payment.
All you need to do is send a message to the freelancer, to briefly explain the mistake and ask her to use the "refund" button in the freelancer-side interface to refund the amount of the mistaken bonus payment.
Any honorable freelancer will immediately refund the money to you. (A dishonorable freelancer will not.)
An honorable freelancer would have immediately contacted the client to ask why the double payment and offer to refund one of them, but maybe this guy is at least "semi-honorable."
You make an excellent point.
Although it does depend somewhat on the timing.
I think in most cases, a double payment would be obvious, and a freelancer could ask the client: "Did you mean to send two bonus payments?"
But if a freelancer is doing a lot of work for a client, and the bonus payments were spaced apart, separated by the submission of new work, then it could look like two separate, legitimate payments.
When I receive bonus payments from clients, they are just something the client sends, without any discussion. So I'm always surprised and grateful. If I did some work, received a bonus, and did some more work and received another bonus, it probably wouldn't enter my mind that the second bonus was a mistake.
I'll check this with the team and will get back to you with more information. In the meantime, have you also tried contacting the freelancer so that he can issue a refund instead?
As a client, if I sent a small bonus simply as a show of appreciation, and not by any sort of arrangement... And I realized that I sent it twice, I would probably be too embarrassed to ask the client to send the second bonus back to me.
We don't know the size and nature of the original poster's bonus payment(s). So that might not apply.