They do not qualify as a witness
Imam Ja’far as-Sadiq (a.s.) is reported to have said,
“The false witness must be lashed, and the number of lashes is determined by the Imam (or the Islamic judge) and he should be paraded (in the city) till people know him (and may not rely on him in the future).”
The Imam (a.s.) recited the following verse of the Qur’an:
“...and do not admit any evidence from them ever; and these it is that are the transgressors, except those who repent after this and act aright, for surely Allah is Forgiving, Merciful.”
(Surah an-NÅ«r 24:4-5)
(It means that after he repents and reforms, his evidence again becomes valid)
The narrator says that he asked Imam (a.s.), “How do we know that he has repented?”
Imam (a.s.) replied,
“At the place where he is lashed, he must confess in public that he had given a false testimony. And he must seek the forgiveness of Allah. In this way it could be known that he has repented.”
(Wasa’il ul-Shia)