What are the two closest capital cities in the world?

Recently, there was a discussion here on Travel.SE about what are the two closest capital cities in the world. One user argued that it is Vienna and Bratislava. Another one stated either Vatican City and Rome or Kinshasa and Brazzaville. A quick Google Search turned out that there are references for all three claims.
Since I'm always very interested in traveling to special locations, I wonder if there is any authoritative reference that tells me to which capital I have to travel to check this from my traveling-todo-list.
Best Answer
Here's one more candidate depending on how loose your definition of "capital" is:
Nicosia is the capital of both the Republic of Cyprus, the mostly-Greek bit everybody recognizes, and the self-proclaimed Turkish Republic of Northern Cyprus, which has de facto control of the northern half of the island but is recognized only by Turkey. However, unlike Vatican/Rome which are co-located, the two sides of Nicosia are separated by a buffer zone -- and hence the distance between the southern capital of Nicosia and its northern half of Lefko?a is the minimum width of that zone, or 3.3 meters.
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What two capital cities are closest together?
Rome and Vatican City Indeed, the two national capitals closest together are Rome, Italy and Vatican City. Rome completely envelopes Vatican City. It's hard to get any closer than that.What countries have 2 capital cities?
Kuala Lumpur and Putrajaya \u2015 Malaysia getting two capitals is a recent thing. Kaula Lumpur acts as the national capital, whereas Putrajaya hosts the federal Government.Which capital city is actually two cities?
La Paz and Sucre are two cities that have agreed to divide the different parts of the government between them. This division dates back to the late 19th century, when opposing political factions had different aspirations about how the country should govern itself.What are the closest European capital cities?
Bratislava in Slovakia and Vienna in Austria are very close capital cities, being 55 kilometers (34 miles) apart with frequent train connections between them. They're often considered the closest in Europe.The Closest Capital Cities In The World!
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Answer 2
I upvoted @Doc's answer, and then found this link:
What are Europe's two closest capital cities?
Essentially it's going to come down to your definition of a city (and you thought defining a country was hard!).
Their definition is that the two closest capitals in Europe are Rome and the Vatican - and at a distance of 0km, make them the closest in the world.
However, they explain by pointing out that the Vatican City is itself a sovereign state as well as a city, and therefore not a true capital of a country, making Bratislava and Vienna the closest in Europe (for the purpose of their question, however it would make Brazzaville and Kinshasha the closest in the world for the purpose of YOUR question)
However if you're asking the two closest capital cities then as the state 'Vatican City' comprises of more territory than just the city 'Vatican City', then yes, you could argue that Vatican City and Rome are the closest in Europe and the world.
Bottom line
So really, it comes down to your definition, and if you count Vatican City as a capital city, then yes, that's the closest (to Rome), if not, then it's Brazzaville and Kinshasha.
Answer 3
As was already said by others, Bratislava and Vienna are the closest capitals in Europe if you don't define Vatican city as a capital.
The really interesting thing about those two is that they have had close bonds up until the end of WW I, since Bratislava was part of the Austro-Hungarian Empire. Then, for about 70 years, the two cities where separated by the iron curtain. Since the integration of Slovakia into the EU, the two cities are sometimes referred to as twin cities.
Another special thing about Bratislava: Bratislava is the only capital in the world to border two other countries.
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Images: Mathias Reding, Nothing Ahead, Sobia Akhtar, Czapp Árpád