Unless I missed something, you are trying to ferret out cases where sysadmins added the computer to more than one default printer group. It is do-able, but the cost in AD/GC queries is too high to do it in a logon script. IMHO, you should run an admin script periodically to do this.
You could also rethink the use of groups for this as it does not guarantee exclusivity. You could leverage an unused property on the computer object or extend the schema to include a default printer property. You could also do it with an INI file as many do.
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