Why is there a question about krisus applied to a full sister as opposed to a half sister (first line on the daf). Since one is liable for each half sister individually, should he not be liable under each category individually. That is, he should be liable for violating the "daughter of his father" even though she is also the "daughter of his mother". Similarly, he should be liable for "the daughter of his mother" even though she is also "the daughter of his mother".
I think that I understand how Rashi points out that one cannot use a kal vachomer to learn the punishment, however, in this case, why would the individual segment for which he is liable be sufficient. Technically is this not similar to the case in which a person becomes chayav to the death penalty twice? It is impossible to perform the punishment twice so that he is punished once even though he is liable for both.
Hillel (Sabba) Markowitz, Baltimore, MD, USA
The Maharsha explains that the reason for the rule of "Ein Onshin Min ha'Din" is that it is possible that the person has done such a severe sin that the punishment for the smaller sin does not suffice.
Accordingly, even if we know that one receives punishment for relations with a half-sister, and this woman is also a half-sister, it is possible the Torah did not even prescribe a punishment for relations with a full-sister since it is such a severe sin (similar to Chilul Hash-m). This is why a verse is required to teach that one receives the same punishment.
All the best,