I tried locating the popstate.dat.file but could not find it, at least not with my Mac search files

I then looked inside Thunderbird settings, found and found papstate.dat file in the mail section. The file was dimmed, however, and I couldn't access it.

I think I might have figured out what the problem is. I have inadvertently set up two Thunderbird accounts. When I followed your advice, I was able to open the pop folder. I also had a mail folder and opened it to find another pop folder. In both pop folders I found a popstate.dat file. When I went into account settings, I found that I had somehow set up two cbldw accounts. The one at the top said mail as incoming server. The second account showed pop as the incoming server. Now this is what perplexes me. Do I attempt to get rid of one of the accounts? I took both popstate.dat files out of the folders and put them on my desktop. When I went back into the two mail folders, behold, the popstate. dat files were still in the folder, having been altered at the time I took them out of the folders.

I'm really in a quandary as to how to proceed. I'm not sure I can delete the second Thunderbird account without wrecking havoc on my primary e-mail source, and there are a few subtle differences in the two different preferences sections.

Any helpful suggestions?

I didn't try to delete the second account. I just deleted all the check marks in the receiving mail preferences. And also, I think deleting the original files might have done the same thing. At any rate, voila, I haven't received a duplicate e-mail in the last hour. Thank you so much for your help. Sometimes if I haven't explored an app in a long time, I forget about its intricacies. When I delve into a problem, I have to re-educate myself. With your patience, I was able to do so successfully.