Now you see the problems you have when you are trying to use a single-user protocol (POP3) to handle multiple users.  If you have an enlightened ISP, when they put the mail into the POP3 mailbox they will also add a special header passing the original SMTP RCPT TO: address the the message.  If so, MercuryD can (and should) be told what the header is and to use this header for delivery instead of the addresses in the RFC 2822 message body.  From the MercuryD help:

Checking for special headers in messages

By default, MercuryD goes through the standard headers in incoming mail looking for local addresses: the fields it examines are: "To", "Cc", "BCC" and "Received". MercuryD also records the Message-ID of every message it processes and usually will not attempt to deliver the same message twice.

Unfortunately, not all ISPs use POP3 mailbox schemes that will work with this approach: some use a non-standard header to record the address of the person for whom the message was actually intended - for example, "X-Deliver-To" is one that is seen from time to time. If your ISP uses a non-standard header to record the delivery envelope address, you can tell MercuryD about it using the Headers control: type in the name of the header Mercury should examine for local addresses (so, from our example above, you would type in X-Deliver-To). The field is not case-sensitive (so, X-Deliver-To and X-DELIVER-TO are treated as identical) and you can add the colon separator at the end of the name or not as you wish. If your ISP uses more than one special header to identify the local addressee, you can enter multiple header names in this field, separated by semi-colon characters (";"). You must not type any spaces in this field.

If you check the control labeled "Check only in these headers" then MercuryD will no longer examine the standard To, Cc, Bcc and Received headers for local addresses and will not discard duplicate messages. Use this control only if you are sure that your ISP always adds the header to your mail.

Your ISP will usually be able to tell you if they use a special header to identify the envelope address in your messages.

Hello,
I have set up a Mercury 4.52 mailserver with about 20 local users. I collect the mails with the mercury POP3-client from my webhoster and distribute them to local users (basically each user has one address at the webhoster with different aliases). Everything works fine as long as incoming mails have originally had just one recipient.
But if a mail has originally been sent to multiple recipients (no matter if TO or CC) so that my webhoster stores e.g. 3 similar mails for different users there is a huge problem. The mercury pop3-client receives that 3 equal mails for different local users and these users receive 3 copies of that mail each. So they don't only get the mail addressed to them, but also the 2 other mails addressed to the 2 other users. In the end there are 9 mails instead of 3 in the mailboxes, 3 mails each (one right and two mails addressed to the other users).
I can see in the message headers that the recipient in each of the 3 mails is correct (3 different and right ones). So why is it also copied into the 2 wrong mailboxes?
There are no filtering rules or alias-misconfigurations that could cause that strange behavior.

Regards, Thomas