If you are using Outlook 2016 (version: 16.0.6568.2025) and configured the email client to download mails using POP3, you might face some issues which will delete all the emails from server inbox after outlook downloads those and/or create duplicate items of the downloaded messages.
Microsoft has shared an workaround to resolve this issue. If you are using that specific version of Outlook 2016, continue reading to know more details of the same and fix the issue temporarily while Microsoft is working on a permanent fix.