Shevuot (Oaths) 32 - Multiple Oaths
If one adjured the witnesses five times outside the court, and they denied knowing testimony, and then they came to court and admitted that they did know the testimony, they are not liable at all. A denial issued outside a court is not legally significant. However, if they denied knowing testimony in court, they are liable for each and every time they were adjured. Their single denial applies to each adjuration separately, and thus they are liable to bring five offerings.
If he adjured them five times, this time in court, and they denied knowing testimony in court, they are liable for only one oath. Rabbi Shimon explained, "Why? Because, after denying knowledge of the testimony in court, they cannot retract and confess that they do know the testimony." Their denial in court of knowing testimony is itself treated as testimony, and the rule is that once witnesses give testimony in court they can never retract of alter that testimony. Therefore, for the last four oaths they are no longer eligible to give testimony.
Art: William Morris Hunt - Justice Lemuel Shaw