I know Brazil is very large and has various accents and pronunciations, depending mostly on the region are native to, so I have 2 questions:
1) Is there a standard/ common/general Brazilian accent that is not unique to a single region?
2) Which accent is used by Duolingo? Is it the standard one (if it does exist)?
I wouldn't say that there is a standard accent here in Brazil. I mean, if two individuals, one from the North and the other from the South, are having a conversation, they will undoubtelly NOT try to speak in this "common/general" accent. As well as we don't have a "Standard Brazilian-Portuguese accent" that is spoken on the news and in formal situations.
There is a "neutral" accent that is used by Course Books and by Duolingo as well. But no one actually speaks like that. It is too neutral and totally not natural for us to speak like that. But I'd guess it comes closer to the accent spoken in some regions of São Paulo...
I know when I was learning Portuguese in school, our two instructors each had a different accent: one from Rio ("Carioca?") and one from São Paulo ("Paulista"). I had the impression those two might be the "standard" accents.
1) No (some could argue the Sao Paulo's one is, but I disagree) 2) I believe the Sao Paulo one.
Though whatever Portuguese you speak is mutually understood all over the country.
I am Brazilian and i disagree Sao Paulo have such a peculiar accent, the "neutral' accent is from Brasilia
Btw, to be able to distinguish the accents, here are some personal tips I have gathered...
São paulo "Paulista" and Paraná "Paranaense"- The r is generally spoken as the american r and "-te" is pronounced as "ch" in "chess".
Porta - /pɒɹta/ and Leite - /leitʃ
Rio "Carioca" and Santa Catarina "Catarinense"- r is usually pronounced as french r and s is usually pronounced as "sh"
Porta - /pɒχta/ and Três - /treiʃ/
Rio Grande do Sul "Gaúcho" (<3) - Most consonants are pronounced as "they should".
Porta - /pɒrta/ and Leite - /leite/
Northeastern "Nordestino" - I believe most consonants are also "pronounced as they should"
Norte and Centro-Oeste - I don't know
Disclaimer: these are just generalizations and of course some regions within the general regions I mentioned. Also don't feel offended if I generalized you with someone from another region, this is all just my opinion so of course there might be some things that are wrong.
1) No. 2) It's an artificial accent mostly used in the media, but I think it is very close (or attenuated version) to the ones spoken in Espírito Santo and Brasília (it has features of some accents).