Yuck!
Umm.. lots of solutions come to mind, but first I must ask:
What problem are you trying to solve?
Why do you want 4 copies of each email in this way?
It really sounds like you need a different email service that supports shared mailboxes.
If you had access to the servers some rules based stuff with procmail, adding a header, and dropping duplicates based on that header to kill loops could work, but it's ugly.
Is there a reason you can't create a user5 that's really a forward and sends email to users 1-4?
A variation on that, what if every user was a copy+forward to user5, and everyone had user5 configured as a second shared IMAP account in their configurations?
Some email services will let you have all of those users login to the same physical mailbox.
I guess the big question for me then is can they share a copy of the email, or do they all need a separate copy of the email to work with?
Is this a situation where an email based ticketing system might really be needed?
That circles me back to what problem are you trying to solve?
More detail about why this is needed would probably lead to a better discussion and more potential solutions.
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