If you have MailScanner running in the background, one issue you can run into is duplicate emails. Try the below to see if it fixes your issue:
Receiving duplicate or repeating e-mails in your inbox.
In '/etc/MailScanner/MailScanner.conf' set 'Lock Type' to 'posix' as below:
# How to lock spool files.
# Don't set this unless you *know* you need to.
# For sendmail, it defaults to "flock".
# For Exim, it defaults to "posix".
# No other type is implemented.
# Lock Type = flock
Lock Type = posix
This resolved the duplicates issue for me.