Do they have the same computer name? If not, then they shouldn't be receiving the same GUID.
Before you install the next agent, make sure the computer's name is different from any of the other computers on the network.
Check the computer's registry to make sure the OEM that sent you the computers didn't install an Altiris agent. If they did, you'll have to remove that information from the computer prior to installing your agent.
This document has a script that will correct shared GUIDs:
http://www.symantec.com/business/support/index?page=content&id=TECH212345