Great question! It is a duplicate within the records, but, it is not a duplicate for the purposes of indexing. You index this because it gives the researchers a chance to locate the document in a search and compare the two when they find the other one. This falls under the General Indexing Guidelines of Crossed Out, Corrected, Cancelled or Voided Information.
A duplicate in indexing must be a duplicated photographic image of the same image in your batch.
okay, thanks so much!