Until a few years ago, birth in Northern Ireland conferred citizenship in two states: the UK and Ireland.
Was this unique in modern history, or are there other similar examples?
This would require that a country grant jus soli to an area outside its control, which often involves a territorial dispute, as Nate Eldredge's comment mentioned. Many territorial disputes are over small, often uninhabited areas, but some are not.
One territorial dispute that comes to mind is the Falkland Islands -- Argentina is jus soli, and the UK was jus soli until 1983 (and British overseas citizens were later granted full British citizenship). So someone born in the Falkland Islands in 1983 is now both a British citizen and Argentine citizen, regardless of the status of the parents, according to each country.
Another example is the territorial claims in Antarctic, of which the UK, Chilean, and Argentine claims overlap. Argentina has jus soli; Chile has jus soli except that it is not automatic for children of transient foreigners, but they may opt for Chilean citizenship; and the UK had jus soli before 1983. So someone born in the area overlapped by all 3 claims will have Argentine citizenship and can opt for Chilean citizenship (though they would likely only opt for Chilean citizenship if they were part of a Chilean family), and if born before 1983 will also have British citizenship.
It is possible that, if and when the territorial dispute is resolved, there might be some provision regarding the citizenship of people born during the dispute. Perhaps a country giving up claim on a portion of territory will retroactively invalidate the jus soli citizenship of people born in that area. Or perhaps the people will have to choose one of the two jus soli to keep. Or perhaps they will be able to keep both unless they choose to renounce one. It is up to the countries involved.