With the Admin Tool you can prevent the opening of multiple sessions with the same user by using the "Allow only one session per user" feature.

Click on the server tile. 

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Click on the Group Policies (GPO) tile.

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Two settings are available :

Select "Only one session per : The second session will capture the first one will allow you to recapture a user session from anywhere by using the same username / password.



Select "Only one session per user: The second session will be logoff" if you wish to prevent any user from connecting multiple times using the same username / password.




However for some reasons you might want to be able to remotely log off the connected user if someone tries to open a new session with the same account. This is possible by using Group Policy. Click on the start menu, and execute "gpedit.msc", or click on the Group Policies in the Admin Tool under "server administration tab" as shown above.

Warning : Group Policy are not available on Home/Family/Basic version of Windows.

Then locate and enable the following GPO : 

Computer Configuration\Administrative Templates\Windows Components\Remote Desktop Services\Remote Desktop Session Host\Connections\Restrict Remote Desktop Services users to a single Remote Desktop services session.