Create an Incident or Service Request Field that won't allow duplicate entries?

Create an Incident or Service Request Field that won't allow duplicate entries?

The manager requested a field be created that allows the technician to "rank" the open tickets in terms of priority according to the customer's request (if 10 items, then rank them 1 thru 10).  I can easily create a new Incident or Service request field allowing numeric values but is there a way to restrict duplicate entries for fields?

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