Midrasch zu Wajikra 13:58

10) (Vayikra 13:58) ("And the garment, or the warp, or the woof, or any article of skin that you shall wash, and the plague-spot depart from them, then it shall be cleansed (i.e., immersed) a second time and it shall be clean.") "And the garment … that you shall wash": I might think (if not for "and the plague-spot depart") that it is sufficient to perform the mitzvah of washing (and that it need not be rubbed thoroughly); it is, therefore, written "and the plague-spot depart" (i.e., to the end that it depart or that it dim to the third degree [and if he sees that it does not depart, it is quarantined for a second seven days]). If only "and it depart" were written, I might think from this side to the other side; it is, therefore, written "from them" — until it is entirely uprooted from them.

11) "then it shall be cleansed a second time and it shall be clean": the second (i.e., this cleansing), to cleanse it from tumah (i.e., ritual immersion); the first, (in verse 54), to quarantine the plague-spot (if it does not depart).