Bava Batra 127 - Questions About Firstborn
A man is believed to say about his son that he is a firstborn, even if it was presumed to be otherwise. The phrase "... he must recognize the firstborn ..." is superfluous, and it is interpreted to mean that the father can identify the firstborn to others, that is, help them recognize the firstborn.
If two sons were born in hiding, and it is not clear who is the firstborn, they can appoint each other an agent against other brothers, and together win the portion of the firstborn - so did Rava advise. They told Rava that even though it sounded logical, the Torah insists on a definite knowledge about a firstborn, and that Rabbi Yannai explained this to be true only when the firstborn was known and then mixed up. The next day Rava publicly announced: "The statement that I made previously was an error on my part."
Art: Léon Augustin Lhermitte Woman with Child and Two Children