Makkos, Mishnah 8. This Mishnah is a continuation of the idea explained in the previous Mishnah: we learned there about a nazir who violated one of the prohibitions of his nezirus several times, that if he received a warning before each and every time, he is liable to lashes for each one. Our Mishnah continues and extends this principle to additional prohibitions.
The tumah of a nazir:
A nazir may not become tamei. If he kept becoming tamei by touching a corpse again and again throughout the entire day, he is liable to only one set of lashes, on the basis of the single warning he received. But if someone said to him "al titamei, al titamei" - do not become tamei, do not become tamei - and each time he became tamei, he is liable for each and every statement of "do not become tamei," and for each and every instance of tumah.
Cutting his hair:
Another prohibition that applies to a nazir is the prohibition against cutting his hair. If he spent the entire day cutting his hair, he is liable to only one set of lashes. But if someone said to him "al tegalach, al tegalach" - do not cut your hair, do not cut your hair - and each time he cut, he is liable for each and every time.
From the laws of nazir to the prohibition of shaatnez:
From here the Mishnah moves to the subject of kilayim: one who wears shaatnez, the forbidden mixture of wool and linen, and wore it the entire day, is liable only once - for the first time he wore it after he was warned. But if that person said to him "al tilbash" - do not wear it - and he removes it and puts it on again each time, he is liable to separate sets of lashes for each and every one.
Two stages in the Gemara's discussion:
At first the Gemara says that he does not need to remove the garment entirely; rather, since he pulled his head out of the opening in the garment and put it back in, this is considered removing it and putting it on anew.
In its conclusion the Gemara establishes that no actual removal is needed at all: if enough time passed between the warnings that he could have removed the garment and put it back on, he is liable for each and every one of the warnings, since sufficient time had passed.
The precise reading of the Ran:
The Ran emphasizes a point: although ordinarily an actual act is required in order to be liable, here it is enough that at the outset there was a real act of putting the garment on. That act suffices to count as the act for all the warnings in which the person said to him "al ta'avor al shaatnez, al tilbash shaatnez" - do not transgress the prohibition of shaatnez, do not wear shaatnez. Each time the amount of time passes in which he could have removed the garment, the act performed at the outset is considered as though he performed the act right now.
In summary: in this Mishnah we learned that repeated warnings create liability for lashes for each and every one - whether in the case of a nazir who became tamei, a nazir who cut his hair, or one who wears shaatnez. Regarding shaatnez, the Gemara explained that there is no need to remove the entire garment, and in its conclusion that no actual removal is needed either; it is enough that enough time passed in which he could have removed it and put it on again. And according to the Ran, the initial act of putting it on is considered a renewed act at each and every warning.