The jam could be it's sending emails, and it takes so long that the next "send email" task starts running over top of it and causes a lock up.
I would increase in the email settings the batch, and increase in schedule tasks the time between sending emails (just search for"mail" and you should find it).
Otherwise you're email client may be slow.
As for duplicates, there shouldn't be any unless the user is clicking multiple times or something. What version and hot fix are you on?