If the prior sale was a cash transaction, how do you know it was arms length and at true market value? What happens if they did a straw deal $100,000 above market value just to set up the appraiser for the second transaction? Of course you can prove all of that with true comparables that are not the subject, which is what a real appraisal would do using comparables that are not the subject in the first place. If one wants to grid the subject as an additional comp (4th or more), have at it, but yes it is redundant, or just another way of explaining that prior transaction of the subject.