MailScanner: Receiving duplicate e-mails in your inbox.
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:
PROBLEM
Receiving duplicate or repeating e-mails in your inbox.
SOLUTION
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.