Does the recipient actually receive the same message twice, or is it that the sent folder has two copies of the same message? Who is your email service provider? What are the current Copies & Folders settings for sent messages for the affected account in Tbird?

I sent myself a test message and did receive 2 copies of it. My email provider is gmail. Copies and Folders has "Place a copy in Other: Sent mail on <email address>". Thunderbird version is 68.9.0, build 20200602112704.

Disable that option for Gmail because Gmail's outgoing server automatically places a copy of sent messages in the sent folder.

Thanks. That does seem to solve the problem -- I sent myself another test message and received just one copy.

You're welcome.