just for your information, I already found a work around for this behaviour... Maybe you have a technical explanation for this and/or another/better solution
Our Customer has a group of 4 people which write offers via mail and everyone has to take notice of every offer.
So one user created a shared folder to accomplish this. Further all created a rule which moves all outgoing mails with a specific subject to the shared folder.
Now all of them can see all offers. But also the recipient retrieves the exact mail 4 times. Even the message-id is the same and also the sender.

Work around: I changed the rule so that it forwards mails to a 5th account. In this account I created a new folder and shared it to the other 4 people.
Then I created a new rule in the 5th account to move all incoming mails with the subject to the new folder.

Server: 14.0.1
Client. 8.0.3HP3 - 14.0.1