Further to the discussion in Double posting, this has happened again to me here. I'm convinced this is a bug rather than user error.
I clicked twice submit because the monastery reacted extremely slow and it looked like the post didn't get thru.
(Probably because of my provider or maybe because the reply was interrupted).
Submitting an additional unique ID of the form (aka nonce) could help blocking such duplicates.
See also Avoiding accidential reposts
(addicted to the Perl Programming Language and ☆☆☆☆ :)
Je suis Charlie!
Unixish Shell Scripting languages
Other (post in comments)
Results (442 votes). Check out past polls.