South Africa has also recently started allowing people to acquire a second nationality, but requires an application to be made for permission to do so. And Australia changed its law a few years ago that simply lifted the bar on acquiring a second nationality, and I don't think any application for permission is needed.

In the middle of all this seriousness, I think it's time for an anecdote about dual nationality, told by Bill Bryson:-

An academic and his family, who had lived in the US for some years returned to the UK. All but their youngest child, aged 4 years, had UK passports. That child had only a US passport but no visa to live in the UK, and hadn't been registered as British.

Asked by the immigration officer, “how long will you be staying in the UK?”, the academic's truthful answer is one year.

“What about the American child?” queries the officer. Without a visa, this child can only enter as a visitor, staying for a maximum of 6 months. After some discussion the officer says he will have to discuss this with his supervisor.

Returning, he says to the academic, “My supervisor will come and ask you how long the child will be staying in the UK. When he does, say two weeks.”

The supervisor duly comes over and asks the question. The academic duly gives the answer "Two weeks".

“That’s OK then,” he says, stamping the passport. Almost as an afterthought he remarks, “While you’re in the UK, it wouldn’t be a bad idea to get the child registered, just in case you decide to stay longer.”