Each time an install of the application is initiated and authenticated to your account a check is carried out to see if the computer is already on the account and has backed up previously.
This check is based on a Windows serial number. So if you have backed up some files, then wiped your hard drive and reinstalled the application it will appear as though there are two devices on your account.
This shouldn't be a problem though as you can re-assign your backup license to the newly listed device by visiting http://my.Mypcbackup.com and clicking MY DEVICES in the left sidebar. Here you will see a table listing all your devices, click the 'Add+' button next to the device you wish to move the license to.