If you haven't read up on what is called worthless doubling. Here is a good start. http://doubleddie.com/144801.html There is a lot of good reading on Wexler's site. Including good photos of real doubled dies. http://doubleddie.com/144801.html
@Danomite is correct. The "S" cents from 1968-1974 were also notorious for die chips inside the mint mark.