1. The name and date of registration for the machine are provided as a reference. If in doubt, it's probably safest to choose the oldest machine in the list.
2. Occasionally Windows Update will cause the same hardware to be seen as a new machine, see point 1 re: dates.
3. Generally multiple installs of X-Plane on the same system are seen as the same Hardware-ID.
With those three rules in mind you should be perfectly safe to have another copy of your products on the same hardware.