“Your son/daughter is the exact spitting image of Grandpa Fred or Aunt Betty.”
Obviously, these people don’t understand how the reproductive process actually works. I can see bits and pieces of parents and grandparents in each and every one of my children, but mostly I can see completely unique individuals. Over time, they grow, learn and change even more into combinations of nature and nurture. I can show you old photos of Grandma Wilma and Uncle Barney that are similar to the children, but there are no clones in this family.
I just reply, “Spitting? Eww! That’s nasty!” It stops that particular cliche right quick like.
You have a quicker wit and more courage than I have.