So what if the song is relatively modern and copyrighted, and what if there are several versions of the song because folks were unaware of the author? Examples:
"Dark Island" - but although there are several sets of words, I haven't heard of variants.
"Wild Mountain Thyme" - although this may just be one individual's misremembering of The Braes of Balquidder, it still has variants.
Hamish Henderson's "51st Highland Division's Farewell to Sicily" - there is the one the way he wrote it (in Scots) and an anglicised version. I know there are others, I just can't think of them at the moment.
Of course, this is probably a stupid question, because that's how songs get into oral tradition. People forget who wrote them and that there's a "correct" way to sing them.