I don't see any reason why a subject once treated by one artist should thereafter become taboo to all others. Think of all the great works of art, how many of them are new or derivative treatments of previously-covered subjects, especially religious one.

A question might arise if the new sculpture were to prove so excessively derivative as to be mere pastiche. It's a bit like copying someone's arrangement of a song - a very grey area, the more so because while a performance is ephemeral a sculpture may last for millennia.

All I would ask of a new sculpture is that it be interesting, and preferably pleasing, in itself. Of the world population only a few people will see the Copenhagen original, another few will see the Folkestone statue, if each of these groups is pleased then I see no problem, though I would not like every port to have one as I doubt that the standard would be maintained.

One thing I like about the proposal is that they are intending to use a real person as model (38 yo mother of 2) rather than some plastic celebrity.

If the concern is that repeating a subject somehow cheapens it, well I grew up close to several mermaid 'sculptures' (Victorian, cast iron, and identical apart from their graffiti) and suggest that they are in no way spoiled by the later Eriksen statue.