Outlook app on mobile devices (both Apple and Android) sends duplicate meeting invitations every time, every user. I cannot find the reason. It started to happen recently. I have duplicate items in "Sent" and recipient gets duplicate messages, and there are duplicate calendar entries. Messages headers differ: one has message id ending with "namprd06.prod.outlook.com" and the other with my domain name. Other apps do not have this problem like iPhone native Calendar app sends invitations correctly. I use Exchange 2016 CU20 Version 15.1 (Build 2242.4). Planning to upgrade to latest soon.
