The gemara (Baba Basra 31b) has a dispute between Rav Huna and Rav Chisda how to handle two pairs of witnesses that contradict each other's testimony, e.g. one pair of witnesses say Reuvain borrowed $100 from Shimon and one pair of witnesses says that Shimon was with them all day and never took any loan. Rav Chisda says that since one of the two pair are definitely lying, we assume that both sets of witnesses are liars until proven otherwise and we can no longer accept any of their testimony going forward. Rav Huna disagrees and says that since we don't know in fact who the liars are, both sets of witnesses retain their believability going forward and can testify in other cases.
What is the focal point of the dispute?
Achronim debate whether the principle of chazakah resolves underlying doubt (birur) or is just a legal fiction (hanhagah). For example, the principle of chazakah tells us that where a mikveh was measured and found to have the proper volume of water, we assume that the status quo of the mikveh's kashrus continues until proven otherwise. Does that chazakah mean that we have no doubt that the mikveh has the proper volume of water until we measure it and discover otherwise, or does the chazakah simply mean that although there may be a doubt as to how much water the mikveh actually has, we legally must accept it as kosher until proven otherwise? Has chazakah resolved our doubts, or told us that the mikevh is legally acceptable despite our doubts?
Shu"T Oneg Yom Tov (Y.D. 71) writes that this issue underlies the debate between R' Huna and Rav Chisda. Witnesses are assumed to be testifying truthfully until we know otherwise -- they have a chezkas kashrus. If chezkah means that there is no doubt as to the veracity of the witnesses' statements, then if one of the pairs is definitely lying, clearly we do have a doubt and must invalidate both pairs. But if chazakah is just a legal fiction that allows an assumption of status quo kashrus in spite of our doubts to the contrary, then even if one of the two pairs of witnesses are lying, since we cannot ascetain who the liars are, we cannot invalidate either pair.