Mishnà su Levitico 13:22

Any doubt in negaim in the beginning is regarded as clean before uncleanness has been established, but after uncleanness has been established a doubt is regarded as unclean. How is this so? If two men came to a priest one having a bright spot the size of a split bean and the other having one of the size of a sela, and at the end of the week each had one the size of a sela, and it is not known on which of them the spreading had occurred, Whether this occurred with one man or with two men, each is clean. Rabbi Akiva said: if one man is involved he is unclean, but if two men are involved each is clean.