Artists Know -- or Should Know -- the Difference Between a Series and an Edition

A quirky question, but let's ask it anyway: If an artist decided to paint the same picture more than once -- say, Andrew Wyeth chose to do another "Christina's World" -- would that be legal? Paintings are supposed to be unique works of art, after all. The question arises because some artists do become known for a particular image. Collectors want that picture and nothing else will seem to do. As a matter of law, artists would seem to be in the clear. "As long as the artist owns the copyright to ...