When a computer disconnects unexpectedly, such as a power failure or technical problem, it may take a few minutes for the system to detect that the user has logged out. If your user tries to log back in before the system completes the logout, they will encounter a message indicating that they have "already joined the virtual classroom".
If you notice such a problem, you should know that it is possible to disconnect the user manually so that they can join again as soon as they want instead of having to wait.
By clicking on the user's name, a moderator can manually log out the user.
Note: It is important not to check the box in the notification that opens (see screenshot below), because then the user will be banned for the duration of the class.
Just click Yes and the user will be able to rejoin the class when they reconnects.