eslHQ - already or just?

Either. It depends when you saw it. If you saw it half an hour ago, "just" is fine. But if you mean "at some time before now" then "already".
The answer that is wanted in the test is undoubtedly "already" and 99 times out of 100 the context would need that. But it's a bad question. It is possible to imagine a context where "just" is possible, eg - Fred is a teenager who suffers from insomnia. He woke up at 4am and couldn't get back to sleep so went on the internet and watched a film. At 7am he got up and got ready for school, and went to the bus stop where his friends suggested going to see the same film after school. John says - No, I've just seen it.
To make the item more "watertight" the stem should be elaborated : I've ........ seen the film. I saw it a few days ago. The inclusion of the final sentence rules out "just" so that that "already" is the only possible answer.