You will experience this if you login on different browser also. As each browser has its own cookies the forms authentication cookie is stored for each browser the system will think that you are logged in on another machine. Also the cookie expires by default for 20 minutes. This means that if you close your browser and within this time frame you open it again and navigate to the back end of the CMS you will get a message that this user is already logged in, however you can login with the same credentials.
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