Although Windows 7 has better access control than Vista or XP  if some one already logged on windows 7 then you are trying to login via remote desctop connection you will probably receive that error.

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To allow 2 sessions simultaneously , please download that fix.

INSTALLATION INSTRUCTIONS (READ CAREFULLY)

Extract the files to C:Win7RDPfix

Navigate with windows explorer to the following folder.

Right Click on "install.cmd" and select "Run as Administrator"

Wait for the script to run entirely.

You should receive the outpoot bellow.

WinRdp CMD