Watching movies you have already seen

So many times when I am watching a movie, there comes a time in the movie when I say, "I have seen this one already."
Be it scene or a line of dialogue that jogs my memory, I usually say, "What is the use of watching it again if I have already seen it"?

Even though I did not remember watching it previously, most times I turn it off.
Rather strange really. Don't remember but still turn it off.