Damien, I have two pointers to bug-buddy/bugzilla enhancements that should eventually help when they become available, and one about a workaround you can use now:
Your bug-buddy suggestion (search for and present user with possible duplicates at bug submission time) is already filed as bug 345103. Sorry, haven’t had the time to implement it yet. Feel free to follow it. However, there are two other possibilities:
Olav filed bug bug 330323 about creating an interface for automatically rejecting bugs with known stack traces. That would allow maintainers to provide stack traces (and gnome-versions, to avoid rejecting regression reports) of frequently duplicated bugs and having bugzilla have bug-buddy notify the users that the given bug is already known/fixed. Unfortunately, it’s another one of those haven’t-had-time-yet-things.
You can opt to not receive email for bugs in the unconfirmed state, and instead rely on the bugsquad or other volunteers to triage bugs appropriately. Go to your email preferences to achieve this.