Dupilcate GUIDS on ris install

Hi all
In our organisation we have about 1200 computers every now and then i have to install a
new ris image. all our pc's are called PCXXXXX when i start the image it say that there is an other
duplicate GUID so i continue now it says it will rename the pc to computer6. I go into AD and make sure
the old pcxxx was deleted i also check the remote guids to see if there are aany duplicates, all
ok. so i start agian still the same so now i enter the pcxxx name and force it to accept then go back to computers and delete the new computer 6.
Can anyone tell me how to find the duplicate GUID or how can i get around this with out making a computer6.
they all new service tags

thanks davy