How to prevent receiving double mails

Hi,

We have to users working an a remote site.

They have only 1 pop3 mailbox at their isp. (pop3@isp.com)

Users are user1@domain.com and user2@domain.com

Mails for these users are forwarded to this pop3 mailbox.

When there is an incoming mail with those 2 users in the to/cc/bcc field, we want forwarding only 1 mail (otherwise the users are receiving two mails)

I setup one users with different mail-aliases. Everything works fine but mails are forwarding twice.

(we moved from exchange, and exchange could prevent this …)

please help