If I have a forum post written up and I accidentally press “submit” twice, it gets posted twice.
I’m closing this as “well, duh”. It’s possible to mitigate this, but there are probably dozens of forms on the site that have similar behavior.
It’s pretty unintuitive behavior, and it’s easy to accidentally double-click. Combined with the inability to delete duplicate posts, it leads to ugly-looking forum threads. It should be a simple fix- just disable the button client-side after the first time it’s clicked.