There was an update to Outlook 2016 in February 2016 which appears to have caused a problem where it will repeatedly download messages from the POP3 mailserver. This is NOT a problem with VPOP3, and there is nothing that VPOP3 can do about the problem.
See this link for more information:
In summary, some solutions are:
- Tell Outlook not to leave messages on the server
- Tell Outlook not to delete messages after X days (this is not recommended because it will lead to indefinite mailbox growth with associated performance and storage issues)
- Switch to using IMAP4 (may require an upgrade to VPOP3 Enterprise)
- Remove the February 2016 update
- Switch to a better email client (eg Thunderbird or Windows Live Mail)
The above link has more details on these options (except for the last one).