I once made a duplicate of a painting and painted the second one after I painted the first one. I am glad that I did it that way as I did not have to correct mistakes twice and the second was painted in half the time.
Portrait artist Elizabeth Vigee-LeBrun (1755-1842) was always very pleased to be able to make multiple portraits for her clients - it was easy money. My understanding is that she did the first one, made all the changes to her client's satisfaction and then painted the others.
Here is an interesting excerpt from her memoirs entitled "ADVICE ON THE PAINTING OF PORTRAITS":
http://www.kcwells.com/technical2.htm