This is interesting. It could include better information for dual citizenship. For example, I searched for China, which does not allow dual citizenship at all

This is interesting. It could include better information for dual citizenship. For example, I searched for China, which does not allow dual citizenship at all, and the search results showed the results for the reciprocal relationship country. Because of the complexity of countries reciprocal relationships, I guess this would need to be a 1-1 mapping country to country. I liked you used the term 'Select your current passport:' and avoided the term 'country' because some passports are for territories. So I checked a territory, Hong Kong. There wasn't an option whether this is the modern HKSAR or the old British National (Overseas), or could it be British Overseas Citizenship? These are all edge cases, cumulatively edge-cases for 10s of millions of people, but perhaps they're better solved by local services rather than a general solution. The main problem of dual-citizenship I've seen is when one embassy says they can't deal with a problem, so go to the other embassy. The other embassy says the same. A bit of a no-man's-land.