MailEnable is installed on the source and the target server.
Mail client (Outlook, Thunderbird, etc.) is configured to use POP3 protocol.
A subscription with email accounts has been migrated to a new Plesk server. After migration, all read messages of the migrated email accounts appear as new in the email client and are duplicated.
This is a Plesk Migrator bug with ID PMT-4208 that is planned to be fixed in future Plesk updates.
As a workaround, use IMAP instead of POP3 as a protocol for the mail account when configuring mail client.
Information how to configure mail client can be found in documentation: