Nedarim 8: Sworn to Do Mitzvot since Sinai

Can one take an oath that one will fulfill mitzvot, which are presumably already incumbent on the person who would be taking the oath? What if the oath is to learn Torah, or a specific section of Talmud, for example? Also, more on excommunication, but this time, when the actions take place in a dream - does that excommunication count? And how can it be annulled?)