the setting is in settings>general

Thanks for the response. However, when I uncheck that box in Settings > General > Incoming Mails, it turns off both sound files. When it's checked, it plays both sound files.

You might want to have a look at what it says in use the following sound file. Perhaps you have two listed there.

No, I do not have the Use the following sound file box checked, and that text box is blank. Please see the attached screen shot. Obviously, this problem is not a critical problem, just annoying, and I apologize for taking up your time.

Have you tried setting Thunderbird to use a local file? I think the file suffix needs to be wav. That might eliminate the double sound. I agree that it is not critical, but it would cause me to hit the keyboard with a hammer after a few dozen double dings. :)

It looks like you're using TB 128 or 115, where you have the choice of notifications delivered by the built-in TB option or the Windows system notification centre. If you keep the latter, turn off the sound in Windows Settings, System, Notifications, Mozilla TB ESR. If you keep the sound, turn off the TB sound by unchecking 'Play a sound'.

Thank you, sfhowes. I turned off the sound in Windows Settings, and that fixed the problem.