I saw it. I have seen it.
This is the difference between the simple past and the present perfect, and is one of the trickiest areas of English grammar for non-native speakers.
The word "already" requires the present perfect here, as Vicki wrote. However, if you add a punctual time reference in the past, you then have to use the simple past:
I have already seen that film.
I already saw that film two years ago.
This is because the present perfect is incompatible with specific past time references.
You can say "I've already eaten lunch." but not "*I've already eaten lunch at twelve o'clock." You must say "I already ate lunch at twelve o'clock."