User logged in multiple times Remote Desktop 2012R2

To configure the Restrict each user to a single session setting on the RD Session Host server

1. On the RD Session Host server, open Remote Desktop Session Host Configuration. To open Remote Desktop Session Host Configuration, click Start, point to Administrative Tools, point to Remote Desktop Services, and then click Remote Desktop Session Host Configuration.
2. In the Edit settings area, under General, double-click Restrict each user to a single session.
3. On the General tab of the Properties dialog box, select or clear the Restrict each user to a single session check box according to what is most appropriate for your environment, and then click OK.

You can also restrict users to a single session on the RD Session Host server by enabling the Restrict Remote Desktop Services users to a single remote session Group Policy setting. This Group Policy setting is located in Computer Configuration\Policies\Administrative Templates\Windows Components\Remote Desktop Services\Remote Desktop Session Host\Connections and can be configured by using either the Local Group Policy Editor or the Group Policy Management Console (GPMC). 

Reference:https://technet.microsoft.com/en-us/library/cc754762.aspx