These sentences do not have quite the same meaning.
The first is in the present perfect, which is an 'open' period of time, or in other words, the time up to and including the present moment.
The second is in the past simple, which is a 'closed' or finished period of time.
Here are some examples of typical situations where you might hear these sentences:
1. 'What movie should we watch tonight? How about 'Gravity'?'
'No, I've already seen that movie twice. Can we see something else?'
The use of the present perfect connects the comment to the present moment, and suggests that there is still a possibility of seeing the film.
2. You might say 'I saw the movie twice' , for example, if you are describing a long flight which you took at some point in the past. There was only one movie to be seen during the flight, and they showed this movie twice, and you watched it right through both times. You would use this tense here because you are recounting something that happened at a precise point in the past. This is an example of 'closed' or finished time.