See this Policy
Computer Configuration\Policies\Administrative Templates\Windows
Components\Terminal Services\Terminal Server\Connections\Restrict
Terminal Services users to a single remote session
If you enable this policy setting, users who log on remotely by using
Terminal Services will be restricted to a single session (either
active or disconnected) on that server. If the user leaves the session
in a disconnected state, the user automatically reconnects to that
session at next logon.
If you disable this policy setting, users are allowed to make
unlimited simultaneous remote connections by using Terminal Services.
For Windows 2008R2 the path is...
\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 = Disabled