The story is set in a space ship where a boy is traveling to his parents. To keep himself company he makes a clone of himself. For a while everything is perfect but then the kid starts feeling like the clone is stealing his life and he decides to get rid of the clone by throwing him out into space out of the garbage chute.
The entire story is written as journal entries by the boy and in the last entry we find out that he did successfully throw him out of the chute but its the clone who threw out the original kid.
I read this when I was 7, in 1999/2000.
It was in a collection of stories called Stories for 7 year olds.