That's what happens when people hit F5 in the shoutbox right after they've posted.
It reposts their message.

It happens to everyone that refreshes the shoutbox after they've posted. Just go onto another page to break the loop.

And a mutual gainer wants to gain weight themselves at the same time as the person they're teamed up with does.
A feeder is someone who feeds someone. A feedee wants to be fed and/or gain weight, often with the help of a feeder.
To stop the message repeating, go onto another screen instead of refreshing the page.

I'm hanging about in there, though it remains silent. People are busy with celebrations currently, I suppose. ^_^