NCforAye
(on a PC now)
By and large, the UK is pretty relaxed about dual citizenship. I have dual citizenship myself. Various countries allow it, to avoid individuals getting treated unjustly. It would be another thing to grant dual citizenship to 5m people.
There is a rough parallel with the former Dominions. My great-aunt was born in New Zealand in 1909. That made her a British subject with the same right of abode in the UK as people born there (it was the same across the Empire and dominions). In 1949, the NZ and UK parliaments created their own citizenships, and she opted for the former (she never in fact lived in the UK). Her brother, my grandfather, was also born in NZ in 1906. He emigrated to the UK in the 30s and stayed there for the rest of his life and became a British citizen at this time. He never became a New Zealand citizen (as an aside, he considered himself fiercely Scottish and would have been horrified by the independence movement).
They both remained British subjects, but that status became pretty insignificant quite quickly. The reality was that they had to opt for one or the other.
I understand that people born in Northern Ireland are entitled to dual citizenship. However, I don't understand this is due to any agreement between the UK and Ireland, but simply because Irish nationality law applies to anyone born on the island of Ireland.
I think one nationality is fair enough.