Help Please!- how to set criteria for picklist

Help Please!- how to set criteria for picklist

How do I set criteria for a picklist so as not to duplicate an existing record. I can only assign a job once. The Jobs.Job_Name is a lookup from the Property.Property_Name field. I tried using the Count function, but can't seem to find the correct way do this. Support Access is set for RAD Project. Thanks. :oops: