Users should be able to add the same domain more than once to SMTP routing.
For example in my case:
I have two mailservers running ISPConfig and then another mail server running the mailboxes for users.
I would like to route mail for certain domains from both the ISPConfig server to the mailbox server, why? To load balance with DNS.
IN MX 5 mail01.ispconfig.tld IN MX 5 mail02.ispconfig.tld
Those two then scan for viruses and spam and send it to the mailbox server.