Pesachim 68A

בועל נדה כטמא מת. למאי? אילימא לטומאתם, האי טומאת שבעה כתיב ביה, והאי טומאת שבעה כתיב ביה.

We learned in the continuation of the previously cited Mishnah that the law governing one who had sex with a menstruating woman is like the law governing one who is ritually impure due to a corpse. With regard to what did they formulate this comparison? If we say they made the comparison with regard to their impurity, that they are both impure for 7 days, impurity for 7 days is explicitly written about this one, someone who had sex with a menstruating woman, and impurity for 7 days is explicitly written about that one, someone who is impure due to a corpse, and so there would be no need for the Mishnah to inform us of these laws.

אלא לאו, למחנותם. ומדסיפא למחנותם הוי ⁠— רישא נמי למחנותם! מידי איריא?! הא כדאיתא והא כדאיתא.

Rather, is it not that they are compared with regard to their camps, to teach that one who had sex with a menstruating woman is sent out from the same camp as someone ritually impure due to a corpse? And from the fact that the comparison in the latter clause of the Mishnah relates to their camps, it stands to reason that the comparison in the earlier clause of that same Mishnah between one who experienced a seminal emission and one who became ritually impure through contact with a creeping animal also relates to their camps, and not as we explained it earlier. The Talmud rejects this argument: Are the cases comparable? This case is as it is, and that case is as it is; each part of the Mishnah formulates its own comparison.