2,861. Reuven or Shimon?

Hilchos Nezirus 2:7

Let’s say that someone says, “I will be a nazir when a son is born to person X.” Person Y overhears and says, “Me, too.” In such a case, there’s a doubt as to whether person Y is rendered a nazir. Maybe his intention was to be a nazir like the speaker, or maybe he just wanted to express affection for person X like the speaker did. Doubtful cases of nezirus are ruled leniently.

Hilchos Nezirus 2:8

Let’s say that two people were walking on the road and they saw someone approaching them. One person says that it’s Shimon approaching and the other says that it’s Reuven. The first therefore says that he’ll be a nazir if it's Reuven, while the second says that he’ll be a nazir if it’s Shimon. If the third party turns out to be Reuven, the first person is rendered a nazir; if it’s Shimon, then the second person is a rendered nazir. The same rules apply in all comparable cases. If the third party doesn’t reach them, having turned around and disappeared from sight so that they never find out who it was, then neither of them is a nazir.