I have done both depending on what the situation calls for. I have shot again or knifed them. I had a buck thrashing his head around every time I got close, so I stood 5 - 6 yards away and double lunged him. He expired in less than a minute.

Another time after shooting a deer from the ground, I found the deer down but not out. She was suffering and I thought it was a safe enough situation for me to knife right behind the shoulder. So I did and the deer subsequently expired.

Whatever is conducive to the situation calls for how you should choose that "second shot" I think.