There are multiple ways to fix the Thunderbird duplicate emails problem. Some prominent manual ways are by using the Thunderbird add-on, filter option, and sorting the Thunderbird email. However, the best solution is to get the professional tool such as MailsWare Thunderbird Duplicate Remover which is tried and recommended by industry experts.
It has advanced features such as multiple data selection modes, saving destination option, duplicate emails deleting options, filters, and so on. For more details, click the link attached above and read the complete information.