Thunderbird POP3 installation receiving duplicates

[Ongun Arısev]

Greetings,

I installed Thunderbird for my father on 2 separate computers and use POP3 protocol to access 4 different e-mail accounts on both of these computers. One of these computers is a desktop machine running Windows Vista Business 32-bit edition and the other is a laptop computer running Windows 7 Home Basic 64-bit.

I configured the e-mail accounts to add the sender e-mail address to the BCC field in order to have them stored in inbox for some time. I then configured a filter on both of these computers to copy these mails to local directories (copy because other computer should also retrieve it). Note that the filter works fine so far and mostly the nonfiltered messages from external senders get duplicated.

After using Thunderbird for some time we noticed that it began to generate lots of duplicate mails in the inbox. So far we have tried deleting the popstate.dat file for the corresponding e-mail account and setting the mail.server.default.dup_action  to 1 accessible through  Preferences -> Advanced -> Config Editor after hitting "I accept the risk!"

However, the mails continued getting duplicated occasionally. After the latest updates both of the computers run Mozilla Thunderbird 52.8.0 (32-bit). Since two computers are connected to the aforementioned e-mail accounts both Thunderbird clients are configured to leave the mails on the server for a maximum of 10 days. 

Furthermore, the Thunderbird client on Windows Vista desktop machine does not display the  two of the account names properly in the GUI. The "@p" symbols inside the account name string is displayed as "_" and I really have no clue about this. Please see the names of the first and last e-mail accounts in the screenshot in the following link: https://ibb.co/mF4nEd

I would appreciate if you could share your suggestions on how to fix these issues and determine whether they are caused by the server or our configuration.

P.S: I did not prefer IMAP protocol as the mailboxes are limited to 256 MB of storage.

Best regards,

Ongun Arısev