There may be better approaches, but I'll share what I did for my account that was similar:

First, if this is an IMAP account, rightclick account name in folder pane, select 'subscribe' and unsubscribe from the one that is not used. Second, if that is not successful, log onto your account website and delete the unused account there when thunderbird is not running. Then, locate the profile folder and locate the folder there and delete it. On restarting thunderbird, it may be gone.

Bonjour Je n'ai pas compris votre réponse Je souhaite supprimer le dossier Sent message car il est en doublons avec le dossier Envoyés salutations

I understand. One of the folders is not being used. You can attempt to unsubscribe from that one, or attempt to delete it from first your online account, and then from Thunderbird.

je ne comprends pas votre réponse

My understanding is that you have two folders and you want to delete one. My understanding also is that your email account uses one of them to store sent messages, but does not use the other folder. Your desire is to remove the folder that is not used.

If that is true, and if the account is an IMAP account, one way to remove the unwanted folder is to highlight the account to which it is involved, then rightclick and select 'subscribe' and then untick the box for the unwanted folder.

Let me know if that works.

merci de votre aide , j'ai pu supprimer ce dossier mais je ne comprends pas pourquoi il s'est créée alors que sur les autres comptes non