I think Northern Ireland is unique in that regard, where every person holds (or at least, is entitled to) dual citizenship from birth.

I guess HKers are kind of simultaneously Chinese and not Chinese until they get old enough to form an opinion.

And I'm not sure if they actually have 'dual citizenship from birth', but South Korea has an open door policy to North Korean.. immigrants/refugees.

I think one of the parents of the child has to be a UK or Ireland citizen. For example, German parents resident in and having a child in Northern Ireland, the child is only a UK citizen (and perhaps German? I don't know that part of it)

UK nationality law is exceptionally complex (probably the most complex in the world). Irish nationality law is less complex but inherited a lot of warts from the UK

In general if you’re born in NI and your parents are settled (that is they have permanent residency or are EU citizens with derived status) or are UK/IE citizens you are both British and Irish.

If one of your parents is NI born, it entitles you to have both UK (by descent) and Irish (by Good Friday) citizenship. Couple this with being born in the US, and my kids have 3 citizenships from birth.

And there are many other overlaps like this - such as RoI people can join the British Army.