I can't recreate this bug, is it SMF 2.1 RC2 or the latest GitHub copy?
My test site is running 2.1 RC2, don't think I've upgraded since. It does it there.
It'll only duplicate the shouts if you enable the submit using enter key. And turning that setting off you don't get the duplicate shouts, but you can still submit by pressing the enter key. It's almost like something's doubled up in the settings somewhere.