From time to time, Outlook downloads multiple copies of incoming mail into my Inbox from a range of email servers. I have asked for support from Microsoft. The response from their Community Forum is:QUOTE
The duplication of the messages are most likely caused by your virus scanner integrating with Outlook and interrupting the send/receive process so the message gets downloaded again. Uninstall this integration part of your virus scanner and try again; you would still be sufficiently protected by your on-access scanner part of the virus scanner.UNQUOTE
I can't work out how to implement that suggestion.