(Mishnah): If witnesses say 'these are our signatures, but we were forced to sign', or 'at the time we were minors or invalid witnesses', they are believed;

If there are witnesses [who testify] that these are their signatures, or if their signatures are available elsewhere (we have another document that they signed), they are not believed.

(Gemara) Support (Beraisa): And similarly, witnesses who testified, whether to be Metamei or Metaher, whether to distance or bring close (testify about improper or proper lineage), whether to forbid or permit, whether to acquit or convict - before their testimony was investigated in Beis Din, if they said 'we lied', they are believed [to retract their testimony]. If after their testimony was investigated in Beis Din, they said 'we lied', they are not believed.

(Reish Lakish): [Chachamim] made witnesses signed on a document like one whose testimony was investigated in Beis Din. (Therefore, they cannot retract.)