ConfigMgr tries to merge records for the same client using either Windows authentication of the computer account or a PKI certificate from a trusted source. An obsolete record is created only if this process fails.
Consider the following scenario that might generate an obsolete record:
1. Machine XYZ is not joined to an AD domain or using PKI certificates, is a current ConfigMgr client, and is healthy.
2. Machine XYZ has a resource ID (a unique id) of 1234 and a hardware ID (another unique ID) of ABCD. (In reality, the hardware ID is a long string of fixed length but is short and simple for the purpose of this example.)
3. Machine XYZ is reimaged from Windows XP to Windows 7. A new client is installed during ...