There have been cases, in literature, of more than one copy of a poem existing.....
Seriously, how many poems or books exist in their original form from antiquity? Many copies were made, but they got lost, thrown out, the paper used for something else. Most have been lost, often due to the actions of Philistines, like those that burned the great library of Alexandria.
We're luck that much of anything has survived.
Not even the Iliad. They didn't start writing these epics down until somewhat later, but even then the versions we have now came later yet.