In Outlook when you 'reply all' the sender does not get the email they sent in their Inbox.

In D365 Customer Service when you use Reply All to an Email in the Timeline of a Case, the sent email will appear 2 times, once as the Sent email, then again as a received email.

The Reply All should not include the sending email account/queue in the To or cc of the email so that it works in the same simple way that Outlook does.

Users currently have to delete the sender queues/contacts from the email that is created from reply all to avoid getting their sent email back as a received email.