I think MR BK is correct and a traveling matte was used so that for those moments, the double's arm was in the frame, while the rest of Granger was still there.....sounds complicated to me.....but I haven't seen it for awhile.
It's kind of like Bette Davis in A STOLEN LIFE when in many scenes the body of the double stayed in the shot, but Bette's head was matted in......