Why would they do that when your biometric passport contains loads of other information about you, which is available to the officer and the eGate? That someone should share the same name is only a small part of the puzzle.
I think that the border agencies are simultaneously more competent and less competent than one expects.
Officers can obtain additional information by going to the office, but I suspect not that much information is immediately available on the screen (based on several interactions where even if they were trying to catch me out, they would probably have phrased questions differently).
The expiry date of the document is clearly noted upon it, it is reasonable to expect that a passport be deemed valid until the date inscribed upon it by the issuing authority.
Say the UK decided that some adults could have passports with a 20 year validity period - then the UK would absolutely have to regard those passports as valid for 20 years, but it would also be reasonable for other countries to regard them as valid for only 10 years if they do the same to their own passports.
The other issue is that if a country has a maximum stay period, they may also want your passport to be valid on the last day you could leave plus a further period for leeway. If you had a residency visa/permit for country A in your passport of country B, even if passport B was about to expire, you would generally be allowed to travel to A if visa A had a longer validity
someone said when they return to either country they are a citizen of, they need to show the passport from that country so they always need to have an up-to-date passport in both countries if travelling to or from them.
Passports of the other country don't work because they would think they have renounced your citizenship, so I was told.
I knew I could apply for an Irish passport but not that I was already an Irish citizen.
I hadn't any intention of renewing my UK passport. If the above rules on which passport to show applies, it may not apply between the UK and Ireland due to the common travel area. I had no issues coming back from France recently via Eurostar.
I don't believe the UK has such rules.
The famous example is the US, where it is an offence for a US citizen to enter the country on a foreign passport -
https://travel.state.gov/content/tr...onsiderations/Dual-Nationality-Travelers.html
Most countries will accept recently expired passports as proof of citizenship. If you manage to turn up at UK immigration with a recently expired passport it should generally be ok, they may do some checks in the office.
The problem is airlines don't want to take the risk.
If a country allows multiple citizenship, there is no reason to think that a citizen has renounced their citizenship just because they have another passport. New Zealand explicitly allows citizens to get a "visa" in a foreign passport saying they are NZ citizens. The UK also has this for citizens who don't want to/can't get a British passport, although it costs a lot more than the passport.
Airlines also do not know your citizenships, so if you have a passport which would allow entry as a tourist that would generally be accepted - e.g. a US/UK citizen could use their US passport to travel to the UK. Currently the UK allows US citizens to enter via e-gates without any extra steps - the UK system may or may not know the US citizen is also British, and it is possible that it might send some of these people for manual processing, but the UK doesn't have a register of everyone who is or could be a citizen.
In the future the UK will require an ETA before travel to the UK, but British and Irish passport holders are exempt. I would think that as long as Irish citizens have the right to live in the UK, there will not be any problem for a British/Irish citizen to use either passport to travel in and out of the UK.
But someone who is UK/another country may end up being forced to have a British passport to travel to the UK, if the ETA system does not allow British or Irish citizens (who don't have the passport) to apply with a foreign passport - like the similar Australia/US/Canada systems. On the other hand plenty of US citizens have managed to get ESTAs in foreign, including British, passports and used them to travel to the US. While it is an offence to not use a US passport to travel to the US, there doesn't seem to be any specified penalty for doing so