Nedarim 17: Doubling Up

Can we make two oaths, shavuot, at the same time?  Can we make two vows, nedarim, at the same time?

The rabbis agree that two oaths cannot be made concurrently.  However, vows seems to be more complicated. In the case of nazirut, vowing to become a nazirite, the rabbis suggest that two such vows might extend the nazirite's commitment.  The minimum time to be a nazirite is 30 days.  If a person makes this vow twice, s/he has promised at least two 30-day stints as a nazirite.

There are other circumstances where the rabbis allow a vow to be taken twice, as well.  An example would be when a person forbids him/herself from eating a fruit twice, and then eat that fruit.  The person is then liable to two consecutive punishments.