If you have "Leave copy of message on server" checked and a large amount of email Outlook will download duplicates if it starts the second download before the first download check has completed. Increase the time interval for how often Outlook checks for new email:
- Click File, select Options, and then click Advanced
- Scroll down to the Send/Receive section and click Send/Receive
- Set the time interval for "Schedule an automatic send/receive every" to 15 minutes
A very large amount of mail on the mail server may require more than 15 minutes.
Also, a few more things to check:
- Check your Outlook configuration for duplicate mail accounts.
- Optimize your Rules configuration.
- Also, firewall or anti-virus protection settings could be set to high.