Shevuot 19: Two Paths Diverged (in a Wood)

Finishing chapter 2! If there are 2 paths, one with a known source of impurity and the other ostensibly pure, and you don't know which is which, and he walked both paths, he was definitely impure, but it's not known when he became impure. But what if he forgot where he walked? How does he track when he became impure? Also, what about a person who did the same transgression where it entails a sin-offering, for example, and he did it again, and isn't sure about what h remembered and what he forgot. With a clear dispute among Amoraim.