Mann = man, Man = you? Du, dere = you. What is going on?
"Man" means "one/you" as in, "One eats breakfast in the morning." "Du" is the singular and "dere" is the plural form of "you," usually.
Is one to tell the difference between man and mann from context, or is the n lengthened when it's doubled?
'Mann' would likely appear with an article, so it would either be 'en mann drikker' or 'mannen drikker'. 'Mannen' will have a noticeably longer 'n' sound than 'man'.
I believe 'man' is often used with generalizations, where in English we might say: "People often do so and so".
In English we would also say 'one is drinking coffee' when talking about a nondescript person doing something. (but isn't said in more 'commoner' dialects)