Two phones appearing on android device manager
the factory reset was most likely successful. i'm guessing the twin devices are showing different locations and one or both never move and may show as offline (if both are offline, dead battery). the device is linked to ADM via your google account and an ID generated when android boots up for the first time. after the remote reset, the device will boot, generate the new ID and ask for the password to the google account the device is linked to. as a ROM addict, i know this all too well. there have been times i've flashed 10 ROMs in a day. i used up my device removal limit on google play music very quickly.