Duplicate ticket creation

We are using Rt 4.0.8 with mysql and sendmail

Our RT tickets are created by our users by sending an email to rt@…

Randomly some of the tickets are duplicating on creation. So far I am
unable to find any pattern in this duplication.

Can some one please guide how to troubleshoot this issue. Which logs
should I be looking at besides the mail logs?

I am wondering when the ticket comes into the sendmail queue, how is it
duplicating? The mail queue emptie out instantly, the only log I see is
mail log with duplicated ticket.