It is not at all unusual for an artist to paint many versions or studies of the very same thing. Monet did it often. Munch in fact did do four versions and a lithograph: http://en.wikipedia.org/wiki/The_Scream Three were done in paint and the fourth in pastel. It is common for a theme or idea to become an object of study and for the artist to try it out in several mediums.
Good question.