Please I Need API code for the customized RTC attribute should allow only unique number while creating new workitems

We have create one customized RTC attribute(SDNumber) for the numeric value and also the same attribute(SDNumber) used to different work items within same project area. So i need API code for customized RTC attribute(SDNumber) should allow only unique number not duplicate Number while creating new workitems using particular work item type because that attribute(SDNumber) is already used to another work item in same project area. So please help me to create API code for the same.