When a non-Jew converts, he is treated by Jewish law as a newborn – ger she-nitgayer ke-katan she-nolad dami. One of the questions that the Sages dealt with is whether restrictions on marriage between family members apply to converts, given that we view them as newly born.
For example, can a convert marry his brother’s widowed wife? Does the prohibition of marrying your brother’s wife apply to two brothers who are viewed by the halakha as being unrelated?
The Gemara relates that a convert who was married to his maternal half-brother’s former wife was asked who had permitted the marriage. In response, the convert said “Behold the woman and her seven children; on this bench Rabbi Akiva sat when he made two statements: ‘A proselyte may marry the wife of his maternal brother’, and he also said that the passage (Yona 3:1) ‘And the word of the Lord came unto Jona the second time, saying,’ should be interpreted to mean that only a second time did the Shekhina speak to him; a third time the Shekhina did not speak to him.'”
Rashi explains that the convert pointed to the woman and her seven children, who were the product of such a marriage, which was done according to Rabbi Akiva’s ruling. The Arukh LaNer suggests that he swore on the lives of his wife and children.
One of the explanations put forward by the Gemara to clarify why this convert’s word is trusted is that he included a second teaching by Rabbi Akiva – the one about the prophet Yona. The Ritva explains that our concern is that perhaps the person who has a personal interest in a given case may misinterpret what he heard from the Sage, given the fact that he will be personally affected by the ruling. Hearing a second teaching was considered to be a convincing indication that the individual had carefully listened to the teachings and had shared them in a precise manner.