Our customers sometimes ask something like this: "We've created a POP mailbox for our salesperson, Robert, at robert@example.com. But any mail sent to that address also needs to go to Robert's boss, Mollie, at mollie@example.com. How do we forward the robert@example.com mail to Mollie as well?"
In a situation like that, you should have the original messages go to a forwarding alias rather than a POP mailbox, then let the forwarding alias do the work of sending it to both people.
For example, you could create sales@example.com as a forwarding alias, typing the sales@example.com destination as:
robert@example.com, mollie@example.com
This works because a forwarding alias can forward mail to multiple addresses. Any message sent to sales@example.com would arrive in the mailboxes for both Robert and Mollie.