SQL Update duplicate records

I know I've seen this in the archives but I can't find the solution. I need to update one field where the entire row is a duplicate. For example:

Pallet Date
915 2013-07-17
915 2013-07-19 Need to change to 2013-07-18
915 2013-07-19
918 2013-07-19
920 2013-07-19 Need to change to 2013-07-18
920 2013-07-19


UPDATE MYFILE B SET B.DATE = '2013-07-18'
WHERE
B.PALLET IN ( SELECT B.PALLET
From MYFILE B, MYFILE C
Where B.PALLET = C.PALLET and RRN(B) > RRN(C)
AND B.DATE = '2013-07-19'
AND C.DATE = '2013-07-19')


I've tried several variations of this statement but it always updates too many records. I know there other ways to fix this problem but I have 9 files with hundreds of records so SQL seems to be the best solution.
Any help is appreciated.

Thanks, Bill






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