Bugzilla seems to accept duplicate POST from a browser, then it is really annonying because one can easily duplicate her/his last submit just by pressing "Back" on her/his browser, thus, incrementing a new #bug with the same content.
I too see people commonly submit a new bug, but realize they wanted to add more details to the bug, so they hit back, and submit again (rather than going to the newly opened bug and adding more detail. This could be prevented by dynamically outputting the blank bug form with a hidden form field containing random server-generated token, and only allow one bug to be opened with that token. (Advanced users would have to reload the page if they were in the habit of pressing back, clearing the fields, and entering new data to open multiple bugs rapidly.)
*** This bug has been marked as a duplicate of 42946 ***