[TRiG_Ireland]

ChatGPT doesn't think, and I fail to see how it is in any way relevant to the discussion.

Marking a long vowel with a macron has a long heritage, dating back to Ancient Greece at least. Yes, some other writing systems, such as Greenlandic, use a double vowel.

Finnish seems to use ä, ö and å as independent letters, rather like Swedish and Danish, unlike German, were ä, ö and ü are regarded as a, o and u with a diacritical mark. These do not seem to be symbols which mark vowel length.

I don't know Māori, but the Wikipedia page gives the alphabetical order for the language and does not list the long vowels separately, so I assume that, as with German or French, they're regarded as the standard letters with a diacritic mark added.

[peterashford]

They are indeed standard letters with diacritics added - but macrons are the only diacritical marks used for Māori. Some people do use double vowels but it's less common than using macrons.