I've entered two bugs today, and in both cases duplicate bugs were created: 117388/117390 and 117393/117394. In the first case, 117390 had the comment truncated, but the complete comment showed up in 117388.
Don't use Back/Refresh features in your browser when/after submitting a bug.
I was going to say that I never reload/hit back during a bug entry. Then I remembered recently installing a Firefox extension called Reload Every that reloads all pages every x minutes... 8-p Sorry, and that extension just got uninstalled so this won't happen again.