I also believe the line "A man's gotta do, what a man's gotta do" was in "The
Man Who Shot Liberty Valance".  It has so many Dukephrases you'll lose count,
pilgrim.  The only other movie it may have come from is "Red River", with Wayne
saying the line to Montgomery Clift.  But I believe it to be the former.  I'll
verify to make sure.