Warning! Spoilers!

From the trailer I thought the movie was about an astronaut battling loneliness on the moon.

Sam is about to finish his 3 year contract on the moon and is dying to go home. There is no live feed from Earth, so he has been communicating with his wife and toddler via occasional recorded video messages. Then he unexpectedly got into a crash. He woke up in the infimary was told by his robot he had an accident. Then he ventured outside and found... himself, still unconscious in the crashed rover.

So now there are 2 Sams, and together they find that they are clones with memory implants of the original Sam. After 3 years they'll get incinerated and replaced with another Sam. They realised they share everything they thought was theirs alone, including the wife and daughter. Looking at the big blue planet Earth floating in the distance, he cried "I wanna go home!" Gosh, that's homesickness brought to a whole new level.

To me, cloning humans (not to harvest stem cells but to create another being) is a direct sabotage on humanity, which has layers of diversity, individuality, self-consciousness. Not only would buffets and insurance companies not work, sports be corrupted, but the family unit and social structure be compromised- children share genetic fathers and relationships no longer unique. That's why we play the genetic lottery and reproduce sexually, not by binary fission. Though Nature gave us clones in the form of identical twins for 2 reasons- porn and scientific experiments. At the rate science is advancing, there may be a day when technology overtakes moral understanding and we'll have more than the Jello twins to worry about.