Kesubos 2:3-4

Kesubos 2:3

If witnesses identify signatures as their own but claim that they were coerced into signing the document, or that they were underage or not qualified to serve as witnesses, they are believed. If others identify the signatures as their handwriting, or if handwriting samples are brought from elsewhere, they are not believed.

Kesubos 2:4

If one person identifies both his own signature and that of his friend, and the friend does likewise, they are believed. If each one only identifies his own signature, then each of them needs another witness to join with them; this is the opinion of Rebbi. The Sages say that additional witnesses are not necessary because a person is always believed to identify his own signature.