Some questions to ask yourself: Some symptoms of this may also include computers randomly disappearing/reappearing from/into your WSUS console.
Another symptom is the computer will receive updates, but not report into the server properly (if at all! Each computer on a network requires a SID (Security IDentifier), which uniquely describes that computer account to the rest of the Windows network resources - supposedly not as big of an issue with computers in a domain environment as it is with workgroups.
You should see the computer report into the WSUS server shortly thereafter.
You can use this script to do it automatically for you - either a remote workstation or local (you should only need to perform this step once on each affected PC):
Checked the SID numbers using the PSTools utility (psgetsid.exe) which proved the SID numbers to be duplicates. EVERY computer reports in 2 or 3 times each day and adds a new entry.