But surely you only actually identify as being a national of one nation?
Two points.
On a legal basis, there are countless millions of people around the world who hold passports of more than one country. The UK alone has between 750,000 and one million double passport holders. Then there is the British diaspora. How many Brits abroad hold citizenship of the countries that they live in? Dual nationality is rather more common than you might imagine. I believe that in Europe only Austria and the Netherlands do not allow it.
Some countries allow citizenship based on one parent, even if you have never set foot in the country. The USA allows citizenship if you happen to be born on US territory, regardless of your parents nationality (remember how Trump spun the lie that Barrack Obama was not born in the US?).
More importantly, which nation(s) might a person feel emotionally attached to?
This is where the real "identity" comes from. According to the US Census Bureau, "Irish heritage is strong in America: More than 31.5 million residents claim Irish ancestry, second only to German (43.0 million)." There are hyphenated Americans of all kinds.
How many people in the UK identify as British first rather than from a constituent nation? I bet there are many, despite what it says on their passports, who regard themselves as English despite the fact that in law, there is no such citizenship. It's how you feel.
So, surely, you
can actually identify as being a national of
more than one nation.
Locally, think about people living in Bournemouth who still feel affinity to Hampshire and then the Cherries around the world who like their adopted homes, but still support AFCB. There are a heck of lot of exiles living in the Greater Manchester area who you meet at northern away matches. Should they stop calling where they live home or should they switch their allegiance to United, City, Stockport or Salford?
If you feel it, you are. Identifying with more than one nation is eminently possible.