Users getting duplicate emails from POP

[amandler]

For several days, customers with email clients that are configured to use POP but leave their email on the server for X number of days are getting all the emails left on the server sent to them when they check their mail. A server reboot did not resolve the issue. I'm not aware of having changed any settings. This is occurring with multiple clients, including Outlook, Mac Mail, and Thunderbird. I am not able to determine what the mechanism is that controls how the email clients decide what are new messages and what are "left on the server" messages, but it's not operating correctly. Any help much appreciated.