I've find a workaroud by modifing the file mysqli_native_moodle_database.php.
The following code line was the original one, in the get_tables function:
The following code line was the original one, in the get_tables function:
$sql = "SHOW TABLES LIKE '$prefix%'";
I've changed it as follows and the problem disappeared:
$sql = "SHOW FULL TABLES LIKE '$prefix%'";
But after the change, a new mdl_backup_ids_temp and mdl_backup_files_temp tables were created.
Then many tables with the same name existed after different activites duplications.
Then many tables with the same name existed after different activites duplications.
The restart of the server didn't change the situation (the db is on azure).
But after some hours and a second restart, the tables disappeard!
After that, I've reverted the changes in the php file and the problems not exist anymore! The duplication of an activity is possible and the tmp tables don't exist.
Any type of comments are appreciated