Gmail already saves a copy in the Sent folder. Go to Thunderbird Account Settings-Copies and Folders and tell Thunderbird to stop saving a copy also.

Yes this works thank you but I don't understand why the duplicate sent items have only started happening in the last few days - they have never happened before and no settings have been changed

I guess you have been lucky since this has been the way it is for as long as I have used Thunderbird with Gmail or any other provider that saves a copy of sent messages for you. Maybe the update activated the Thunderbird save option. Who knows. Now it works. Go enjoy the day.

I have the same problem but it doesn't seem to be related to gmail. What's more, I'm not convinced it has been happening all the time (i.e. the duplicates haven't been accumulating steadily over time). It just looks as if suddenly all the messages in 'Sent' got doubled up.

I need to investigate further to prove that, of course.