Edited Netaddress Email Keeps Duplicating

In Outlook 2016 I have two IMAP accounts, one to exchange server that allows a read email subject line to be edited then saved without problem. The other is to a Netaddress email server. When I follow the same edit and save procedure in a Netaddress email it duplicates over and over, the only way to stop it is to close Outlook. So is this an undesirable feature of Outlook or is it a Netaddress problem or both?