We've added a couple layers of protection to keep commenters from submitting their comment multiple times.
This happened most frequently by accident, when the system didn't appear to respond to the submit button, so the commenter just kept clicking. (Hey, if it didn't respond the first six times, maybe if I just keep on clicking it eventually will!) The problem was exacerbated by a missing database index (now fixed) that caused comment submission to be slow on sites with lots of comments.
We now have in place two layers of protection:
- On the server side, if we see sequential submission of identical comments, we ignore all but the first one.