I'd suggest to switch from Auto-Cron to a usual cronjob. The duplicate emails will probably move away then too ;)
For more info regarding the cron job (or scheduled tasks when you're under Windows and not Linux) check out the email related wiki articles: http://www.osticket.com/wiki
Alternatively, instead of moving emails delete emails. Otherwise it can happen that the email gets fetched twice before the mail is then moved to the imported folder.
