Duo should accept "euro" as the plural, according to [https://en.wikipedia.org/wiki/Euro#Linguistic_issues]
On the other hand, this would seem to indicate that "eŭroj" is correct: https://en.wikipedia.org/wiki/Linguistic_issues_concerning_the_euro#Esperanto
Correct too, looks like the EO plural is always "eŭroj". The English plural should be "euro" according to the institution that actually issues the things, with "euros" having some support as an alternative, and most common use depending on the country. I'd go with the Irish "euro" plural as carrying the most weight, Ireland being the only English-speaking country in the eurozone.
I'd argue that, in English, how people use the language is a more important indication on what "should" be used, rather than what any institution tries to claim. And I've yet to meet (or hear of) a person who would use "twelve euro" instead of "twelve euros".
I know the part of Ireland I'm in, at least, "euros" sounds very wrong. It's a 2 euro coin and a 5 euro note.
Fair point, but in those cases 'euro' is a modifier for 'coin' or 'note'. It's not the main noun and thus the situation is markedly different. Which doesn't mean that I'd dispute your claim of 'euros' sounding odd. Just that I too would say 'a two euro coin'. It's only when 'euro' is the main noun (say, in 'a coin worth two euros') that I'd use the s-plural.
A little annoying, and certainly an odd feature of EO, given, at least in Germanic and Romance languages, the equivalents of 'each' tend to act more as adverbs and determiners than as proposition. I wonder where Zamenhof got that from.
I bet he got it from Russian - both the usage and the form. This is exactly how it is used there: po + NUM + NOUN.GEN
Yeah, Russian was one of Zamenhof's native languages, and many grammatical elements of Esperanto come from Slavic languages.
Zamenhof: "Inter la esprimoj 'doni po 2 pecojn' kaj 'doni po 2 pecoj' estas ankoraŭ malfacile diri, kiu estas la pli bona, kaj tial ambaŭ esprimoj estas uzeblaj kaj bonaj".
After studying three languages I have come to dislike my mother tongue at times. "costs" "cost"
Duo is trying to correct "They cost €3 each" to use 3€ instead for the English translation. Sure, putting the sign after the amount is standard practice in Continental languages (and so is probably the standard for Esperanto), but in English currency signs are always put before the amount for all currencies, including the Euro.