Jeremy asks: "How many US congressmen/senators have dual citizenship and why is this allowed?"
Ah, "dual citizenship." An expensive way of letting one's hipster friends know how quirky they are, or a nefarious scheme to subvert American democracy from within?!
This is an issue that has been a bit of a boogeyman from both the left and the right, and the issue has spawned several rumors -- some true and some not.
Your first question -- how many members of Congress -- is one with an easy answer: nobody knows for sure. Forbes has estimated it's likely between one to two dozen. Several news outlets have tried to get at a more specific answer in the past few years, and without much luck, given that there is no requirement for members of Congress to publicly disclose dual citizenship. Several notables have called for this to be a required piece of information, particularly Michael Hager, former Director-General of the International Development Law Organization:
"The Congressional Research Service of the Library of Congress should begin to include citizenship (along with the current listings of party breakdown, age, occupations, education, Congressional service, religion, gender, ethnicity and military service) in its published profiles of each new Congress."
(The Hill, April 2015)
As a side note, some of the concern has been fomented by an urban legend about 89 members of Congress holding "dual citizenship citizenship with Israel." This claim is not accurate.
PolitiFact already ruled this a "pants on fire" claim, so we won't re-hash the same line of reasoning here. Suffice to say, the claim is ultimately based on Israel's "Law of Return," which is a law that allows people of the Jewish faith (with a few exceptions) to immigrate to Israel and become citizens. However, as you can see from the text of the law linked to above, it is not an automatic granting of citizenship to every Jewish person around the world.
As to why dual citizenship is allowed, it's important to understand what dual citizenship is and isn't. Each country has its own rules of who they consider a citizen, and those rules can certainly (and often) confer citizenship regardless of where you live or whether you've filled out paperwork. There's no special process to becoming a dual citizen -- you don't get a special passport or a decoder ring. In fact, you might not even realize you've got citizenship in another country!
U.S. Senator Ted Cruz, for example, was born in Canada and, by virtue of that fact, automatically a Canadian citizen. But by virtue of his mother being an American, he was also a natural-born American citizen. And here's a vital part of the explanation: countries don't conference call about who gets to take precedence on a Canadian-born American. There's no registry of "dual citizens," and neither country requires you to renounce your other citizenships. As a result, Ted Cruz didn't realize he was a Canadian citizen until 2013:
"Because I was a U.S. citizen at birth, because I left Calgary when I was 4 and have lived my entire life since then in the U.S., and because I have never taken affirmative steps to claim Canadian citizenship, I assumed that was the end of the matter,"
Eventually, Cruz opted to take the affirmative step of renouncing his Canadian citizenship, which because Canada is so friendly they have a really easy form to fill out.
So we get to the crux of why it's "allowed"... Simply because there's no good way of knowing.
Say, for example, that the (fictional) nation of Freedonia got it in its mind to confer Freedonian citizenship to a (also fictional) U.S. Senator Bob Rumson. According to Freedonian law, this would be allowed, and there might be no requirement that Freedonia ever informed Senator Rumson of this honor!
So basically, it's "allowed" mostly because it's a designation that other nations can bestow or revoke according to their own laws and customs. Therefore, enforcing such a rule could be drastically unfair to members of Congress who are bestowed citizenship they may not have asked for or even knew about.
This is not to say that the United States is altogether thrilled with the idea of dual citizenship. On its website, the U.S. State Department explains some of the potential pitfalls and concerns that come with being recognized as a citizen of two nations, and specifically notes:
"Claims of other countries upon U.S. dual-nationals often place them in situations where their obligations to one country are in conflict with the laws of the other,"
... but also notes that "A U.S. citizen may naturalize in a foreign state without any risk to his or her U.S. citizenship." So it seems like it comes down to the issue of a difficult-to-enforce problem. Kind of like saying: "Look, we're not super-thrilled about this, but we can't keep other countries from doing their thing, so just try not to get in too much trouble."
This is due to the unenforceability of many laws and treaties that revoke American citizenship on the basis of dual citizenship. From the 1880s to the 1930s, the United States entered into a series of treaties with 34 other nations collectively called the "Bancroft Conventions," which spelled out strict rules for dual citizenship and often led to the revocation of U.S. citizenship upon moving to another country. This was initially meant to prevent Prussians from avoiding military service by naturalizing in America before returning to Prussia.
But then in 1967, the U.S. Supreme Court ruled in Afroyim v. Rusk that a U.S. citizen, whether by birth or by naturalization, could not involuntarily be deprived of U.S. citizenship (though the government could still write statutory requirements into the naturalization process). This stems from a very stern fourteenth amendment to the U.S. Constitution when it comes to citizenship:
"All persons born or naturalized in the United States, and subject to the jurisdiction thereof, are citizens of the United States and of the state wherein they reside."
Simply put, there's no provision after that saying, "... unless they also get citizenship somewhere else, in which case the deal's off!" As noted above, the naturalization process can still include statutory requirements and a person is deemed to have voluntarily renounced their citizenship under various conditions, such as entering military service of another country. But these factors would not be likely to apply to sitting members of Congress.
But this doesn't get to the question of members of Congress who do know of their dual citizenship or actively seek out another citizenship status. This seems more like the kind of reporting that Mr. Hager is supporting (otherwise members of Congress would always be at risk of failing to report an unknown citizenship!).
As an example, then-Congresswoman Michele Bachmann of Minnesota became a citizen of Switzerland in early 2012. Her husband, having Swiss parents, was eligible for Swiss citizenship and successfully claimed that status. As his wife, Congresswoman Bachmann also was deemed a Swiss citizen. Unlike Senator Cruz's experience, Bachmann was well aware of her status and very open about it, as evidenced in this video interview. But again, there's no such requirement to be open about that status.
In sum, we don't know how many members of Congress hold dual citizenship, and that's because there may be circumstances in which they themselves may not know! But it's certainly not as widespread as some misleading Facebook posts have made it out to be. It's allowed because there's no really good way to enforce it, but some people have called for there to be more transparency and reporting in situations where the official has affirmatively applied for another citizenship status or otherwise is aware of it.
