If your POP3 email client (Outlook, Thunderbird, etc) is downloading duplicated emails, you may refer to the following steps to troubleshoot:
Double-check your email account settings in Outlook/Thunderbird to ensure they are correctly configured. Follow through the links below to check your settings:
Sometimes, network interruptions during email download can cause Outlook/Thunderbird to receive duplicate copies of emails. This can occur if the connection drops or if there are delays in communication between Outlook/Thunderbird and the mail server.
Try changing your connection to another ISP (e.g. your mobile broadband), or restart your Internet modem/router to resolve the issue.
Overly aggressive antivirus or firewall settings can interfere with email send/receive and lead to duplicate downloads. Temporarily disable antivirus software or adjust the settings to allow Outlook/Thunderbird to communicate properly with the mail server.
Check if any installed Outlook/Thunderbird add-ons or extensions are conflicting with the email download process. Temporarily disable any add-ons or extensions and see if the duplicate email issue persists. If the problem resolves after disabling them, consider updating or removing the conflicting add-on.