Device manager shows recently used devices with a weird criteria. If you go to the Play Store and click the settings wheel and click settings, it will show each device that Google shows you using recently. Cleaning the no longer used devices can be done here.
Well, you can't delete a device, only set it to not be visible. If they are both the same date, that's a tricky one. I guess you could set one invisible and go back into the "Find" part of ADM and see if it finds it. It will only find the current device (provided you have the ADM stuff enabled on your phone)
What did you do? Did you get a replacement phone? Or fiddle with a custom ROM or something?