How to stop multiple downloads of entire contents of inbox?

First, i cannot fix this. I am writing to share a possible problem. There is a file in the POP account named, popstate.dat. This little file saves the identifier for the last downloaded mail. BUT, if the file is corrupted, or if it is prevented from being updated, possibly by intervention by antivirus or VPN or other software, the last downloaded message does not get updated. Because of that, thunderbird keeps downloading from the beginning. I suggest you look at the file in windows file explorer to see the last date it was updated. that may be a clue for you.

Thanks for responding, David. My search found one popstate.dat file. When it came up in the search, it was time stamped a few minutes before my search, so I'm not sure that helps. Another detail is that I have four different email accounts in Tbird, and this problem occurs in the inbox of only one of those four. The others are not affected.

There should be one in the account folder of each POP account.

OK, I see three in the Mail folder, and the fourth email is in the Imap folder, and its folder does not include a popstate.net file. I've isolated the folder for the relevant email account, and there is a popstate.net file in it. It looks like it was updated within the last 30 minutes. But I just tried Get Mail again on that account, and the emails resumed downloading from where Tbird left off. I just opened that popstate.net file using Notepad. It identifies the email account with the problem in Tbird and lists a couple hundred lines of some sort of identifiers--I presume they identify specific emails?