Good question, Carl. Glad to answer this since I had the same question early on.
Not sure of the details surrounding Schmidt and Walcott, but I do know that Duncan was ruled out on dental inconsistencies early. DNA was not yet complete when this dental comparison was made back in 2010. Many rule outs are based on scientific methods (dentals, physical descriptions, etc) available at the time of request. But investigators see mistakes made too many times. For example: Dental records could have been misfiled, height entered as inches converted in error to feet and inches when put into the data base, i.e. 57 inches becomes 5'7", or the last known weight and height is from a four year old drivers license.
Whatever the circumstance, when there is any doubt investigators like our friend Rick requests that a DNA comparison be conducted once samples are entered. Rick has also shared that there have been instances when even DNA submitted by family members did not match their loved one -- because the family never told the victims siblings that their sister/brother was adopted or had a different father. Sometimes these secrets are reluctantly revealed during his investigation, but he has to consider the possibility that family secrets can die with the parents and make DNA comparisons impossible.
Rick has not shared with me his reasoning behind still pursuing Schmidt and Walcott's DNA. If he does, I will share.
