Mishnà su Numeri 6:2

One who vowed [to keep] a longer naziriteship [than ordinary] and he completed his naziriteship and afterwards came to the [holy] land: Beth Shammai says: [he must be] a nazirite [only] thirty days, But Beth Hillel says: [he must be] a nazirite [the full time vowed as] in the beginning. One who has two groups of witnesses who testify about him, these testifying that he vowed two naziriteships and these testifying that he vowed five: Beth Shammai says: their testimony is divided, and there is no [obligation to perform] naziriteship. But Beth Hillel says: within the five, two are included, so that he must be a nazirite twice.