Is there a "Property Value must be Unique" policy rule in a Property Generation and Validation ( PGV) policy?
Out of the box there is no "Property Value must be Unique" policy rule in a Property Generation and Validation policy, however, as a workaround, it is possible to enforce uniqueness on the samAccountName and build other attributes containing this unique value.
Then add a Property Generation and Validation policy for the attribute where a uniquness constraint is desired, with a rule based on samAccountName.
The only other option would be to develop a custom scripted solution. For more information on this option, please consult Professional Services or Sales.