There's no rule that says you can only write 1 song about a given event. Most writers talk about the same things over and over again, just from different perspectives or in different words. Ayu herself has said in the past, when talking about the trilogy, that she is one such person.

A person who reads only one translation of a foreign text has a tenth as much understanding of it as the person who reads ten translations. This is especially true with Japanese-English translations. There is no authoritative translation.