Is there like an international database of passport details or something? If someone holds dual (duel? oh noes) passports and travels on both, are the origin countries aware? When I search for information on this all I can find is how to apply 
There is no international database of passport details, but there are international standards for making passports, and the latest ones have nifty biometric microships in them, and all sorts of anti-counterfeiting measures.
Whether or not a given passport is valid can be determined only by that passport’s country’s passport-issuing agency. In the US, that’s the Department of State.
If you have dual citizenship and two passports, the only ones who will know that are the countries that issued them. If those two countries have a mutual information-sharing agreement (like most of the Anglosphere and the EU) then both countries may find out. However, having dual citizenship and two passports is not generally a crime, unless there is intent to defraud or commit nefarious acts abroad.
How is a passport “revoked”? Does the State Department send out an all points bulletin to points of entry around the world not to admit this person on this passport?
Once upon a time, I was living in the Czech Republic. My friends had given me a cover to put around my Dutch passport which said ‘Česká Republika’. I thought it was the coolest thing ever. Then we went on a road trip to Croatia. When we reached the Croatian-Slovenian border, the guys there were really confused (no confusion at any other boarder, though, and I’d been travelling around with this thing for a while now). Anyway, they made me get out of the car and step into their little booth, where they proceeded to show me a document that had apparently been issued to them by the Dutch ministry of Internal Affairs so that they could establish whether somebody was really Dutch. It was basically a language test; it said (in Dutch): ‘draw a square’, and then it showed a square, a circle and a triangle and it said ‘point out the circle’. It ended with telling me to read out loud the final page of my passport (the part that says that the minister, on behalf of the queen, respectfully asks that yeddi-yeddi-yeddi…).
So if you’re ever travelling to Croatia with a forged Dutch passport, you’ll know what to do.
Whether or not you are issued a passport is up to the country from which you want one. They might have bilateral or multilateral information sharing agreements with other nations, but there’s no general database of passport holders (thank God - such a database would be a nightmare, privacy-wise). It’s also, btw, up to any country to decide which foreign passports it honors.
Most countries discourage multiple citizenship, and there are treaties aimed at reducing this by coordinating national citizenship law, but it occurs, and it’s not that rare (right now, at least two people who I know personally come in mind with multiple citizenship - one Iranian/German woman and one Greek/German/American guy). Usually, the national authorities will know if one of their citizens holds another nationality - the criteria the other nation uses to determine who is one of their citizens are know, and if, say, American authorities know you were born in Tehran chances are good they might check Iranian nationality laws to determine whether you automatically acquired Iranian citizenship upon birth. It might be more difficult with naturalization, but many nations require candidates for naturalization to renounce their previous nationality and provide evidence of this, so the other country’s authorities will get notice of that.
Many countries will even issue multiple passports to its own nationals if there’s a valid reason for having more than one. I know that Germany will give you two passports for travel in the Middle East because of the policy of several Arab nations of not admitting persons with an Israeli visa into the country.
I assume something like this really happens, at least among nations participating in the applicable information sharing agreement, and I don’t think it would be too difficult to set this up, given today’s computing technology.
During my university studies, I served an internship with a German embassy abroad. I came across a bulletin issued by the German authorities to airports and border checkpoints in the Schengen area (the union of European nations which do not require visas for internal travel among them and issue common visa for people from the rest of the world). Basically, the bulletin said that a bunch of blank visas for entry to the Schengen area was stolen in some German office. The bulletin listed the numbers of the stolen blanks and annulled them.
This may well be the case at many airports - Interpol has an international system so that if you are wanted, it is fairly easy for them to cast a wide net. However, many land borders have no computers, tho most of these are out of the way (there was not even electricity when I crossed from Rwanda to D.R. Congo).
‘Usually’ is probably stretching this a bit far. I have dual Irish and British citizenship, being born in Britain to a British and Irish couple. Neither country would necessarily know enough information to be sure of my status in the eyes of each other - while my mother’s place of birth is entered on my birth certificate, this does not guarantee that she was at the time of my birth an Irish citizen. Likewise, Ireland has no official records of me other than those I have provided in a passport application, and these would not fully prove whether or not I was recognised as a British citizen under British law.
I’d also query whether ‘most countries discourage multiple citizenship’ - it’s certainly not the case here, with British law creating millions of dual citizens such as myself, many not even realising that this even applies to them. (Discouraging the possession of multiple passports is a different matter, of course!)
But do British or Irish authorities actually know about your other nationality in your specific case? I assume they do. While the information you mentioned do not necessarily mean you have dual citizenship, they at least imply that it’s possible, giving the authorities reasons to investigate if they care about your status.
Well, that’s a matter of Realpolitik - multiple nationality is a fact, so most countries have chosen to accept it and get along with it. Nonetheless, there is a general feeling that it is undesirable, because it causes a lot of problems regarding administrative law and private international law. There is a treaty called Convention on the Reduction of Cases of Multiple Nationality and on Military Obligations in Cases of Multiple Nationality to which most European nations are parties, including the UK and Ireland. It explicitly states in its preamble “that cases of multiple nationality are liable to cause difficulties and that joint action to reduce as far as possible the number of cases of multiple nationality” are in the common interests of the participating nations.
No, they don’t have any formal knowledge of my other nationality. And while it is true that there’s enough there to imply that the situation is possible, this is the case for many millions of other people, too, and nobody is going to start checking up on these except in very exceptional and unusual circumstances - certainly not simply when issuing passports.
I’m certainly no expert, but this seems to mainly deal with people acquiring a different citizenship, and not with those who are recognised from birth as citizens of two different countries. Is the latter ever tackled in such treaties?
Browsing through the text of the treaty, I’d say that yes, multiple citizenship by birth is also a matter addressed by these agreements. It’s just more difficult to prevent. If a country grants its citizenship to anybody born to a parent who has that nationality, as many nations do, there’s no way to prevent multiple citizenship since neither father nor mother inherently deserve preference. Demanding that both father and mother cumulatively possess the respective nationality for the child to acquire it by birth is even worse - if that principle were adopted internationally, it would mean that children born to mixed couples would end up stateless, which is really nasty for the person concerned (and tackled by the Convention on the Reduction of Statelessness[PDF]).
This applies to jus sanguinis countries, which use parental nationality as the criterion for acquiring citizenship by birth. There’s also the concept of jus soli, where people born within the country’s territory acquire citizenship by birth. This concept evidently avoids multiple nationality by birth, but it involves the risk of having children whose nationality differs from the country they are most closely linked to if their parents reside, even temporarily, abroad. That’s why most countries which use jus soli don’t use it exclusively. The U.S. relies mostly on jus soli, but also provides possibilities for children born abroad to a parent who is both a citizen to get citizenship by birth under certain conditions.
Multiple citizenship as a result of naturalization is easier to avoid than multiple citizenship by birth, which is why the Convention I linked to contains mostly provisions regarding this. In general. however, multiple citizenship causes a lot of problems and can be disadvantageous to the person concerned (see, for example, the problem of multiple military conscription, which is specifically addressed by the Convention). That’s why it explicitly leaves room for further national measures to prevent multiple nationality (Article 4).
Someone found to be traveling on an invalid passport is in violation of 18 USC 1544, and subject to 10 years in the clink for a first offense.
Interesting, but doesn’t really answer my question.
I’m not trying to be awkward here - I genuinely can’t see how it addresses multiple citizenship by birth, except for the specific matter of compulsory military service. 
Bar owners can get booklets that show pictures of all of the individual states’ driver licenses. I would be surprised if a similar document wasn’t available for border guards. At least for the training phase.
I was trustee of an account in the US and had to distribute some funds. I presented my UAE Driver’s License as ID and they indeed have a book showing driver’s licenses from (presumably) every country. The UAE has 7 Emirates and each one has their own license design - this book listed all 7 with the security features of each.
I’m not saying that your dual nationality by birth is contrary to the treaty. I merely cited the treaty to support my point that multiple nationality, while accepted by most countries as a fact, is regarded as something which should not appear too frequently because it involves a lot of problems, and that measures are taken to reduce it if possible.
With regards to driver’s licenses (in their original function, as a permit for driving, not as a form of ID), there’s also a document called International Driving Permit. It doesn’t involve any additional driving tests; instead, you can get it from national authorities upon request if you have a respective national license. It has a standardized design and is valid only if used in conjunction with the national license. Many nations require drivers from abroad to possess an IDP for driving within their territory - presumably to avoid the necessity of having to know all the categories of licenses and vehicles used worldwide.