Kesubos 2:7-8

Kesubos 2:7

If there are two men and each one claims to be a kohein, they are not believed but if they testify about each other, then they are believed.

Kesubos 2:8

Rabbi Yehuda says that a person is not recognized as a kohein based on the word of one witness. Rabbi Eliezer says that this is only the case when someone contests his claim but if no one contests his claim, then one is recognized as a kohein based on the word of one witness. Rabban Shimon ben Gamliel said in the name of Rabbi Shimon ben HaS’gan that we always recognize someone as a kohein based on the word of one witness.