Tosfos (Makkos 13) writes that if a kohein marries a woman who is both a divorcee, a gerusha, and a chalutza, the kohein would receive only one round of malkos as punishment for violating the Torah prohibition of marrying a gerusha, but no additional makkos mardus for violating the Rabbinic prohibition of chalutza. The Mishne laMelech (Issurei Biya ch 17) derives a general principle from Tosfos: once a person is liable for punishment for violating a Torah prohibition, he/she cannot simultaneously be punished for violating a Rabbinic prohibition. The Pri Megadim (Pesicha haKolleles I:28) disagrees. He assumes that a person could theoretically receive makkos mardus along with regular malkos. The case of a gerusha who is also a chalutza is an exception, because Chazal used the extra “vav” in the word “v’isha m’gerusha” as a basis for the law of chalutza. In other words, Chazal placed chaltuza under the same rubric as gerusha, and you can’t get double punishment for the same issur twice. In other cases where the prohibitions are different, there would be punishment for both.
What is the point of dispute between these achronim? One issue obviously is how to define the issur of chalutza – do we look to the root pasuk as a basis for defining the nature of the issur or not. But what about the more fundamental issue – why should the fact that a person is getting malkos for an issur d’oraysa automatically exempt them from receiving makkos mardus for an unrelated issur derabbanan?