If they say 'these are our signatures, and others say 'these are their signatures', R. Chiyah taught that they cannot add or detract [from their testimony].

Support (R. Yosi - Mishnah): If there are witnesses [who testify] that these are their signatures, or if their signatures are available elsewhere (a validated document that they signed), they are not believed.

If they say that these are their signatures, and others say that these are not their signatures - R. Mana says, this is like a contested document. (We do not tear it, but one may not collect with it.)

(R. Asi): (Our Mishnah said that if their signatures are available elsewhere, they are not believed.) This is only if [we have their signatures on] a document that was contested and validated in Beis Din.

(R. Lazar): [It is even if it was not contested, and - SEFER NIR] it was not validated in Beis Din.

(R. Ila): R. Lazar's reason is, since the witnesses were established to be signed on the document, it is as if the testimony was investigated in Beis Din.