There are certain things that the board wants to be able to rely to in an attempt to streamline its work. That is to say, certain things they want 100% probability are going to be taken care of in the report. I can see that disclosing prior appraisal on the subject as one of those. So, you have to give them that right. Also consider, the appraiser said "he just forgot". But, for the board to verify that, they would have to check his past appraisals. You know how much work that would be for them? They are probably not even set up for that:
1. They have to get a list of all appraisals the appraiser has done over at least the past 4-6 years (min past 1 year plus another 3 years).
2. They have to somehow verify that the appraiser has not done any other appraisals in that time (I don't know how, is his signature enough? ).
3. They find any instances where he has appraised the same property more than once.
4. Then they have to check the give reports to see whether he stated prior work on the given properties.
Maybe it would be simpler just to prohibit him/her from doing appraisals for 3 months.
I think appraisers should have to report all appraisals to the board when they do them. That would take care of the problem, most likely. Just get online and report the appraisal when it is completed.