The rule preventing application of a prohibition to an item which is already prohibited
Prohibitions which begin simultaneously are both effective; neither is canceled: Keritot 14b, 23a, 23b
Applying a stricter prohibition atop a lighter prohibition: Keritot 23a-b
Whether this rule applies to korbanot: Keritot 23a-b
Cases
Applying the prohibition against leaven on Pesach on tevel [untithed produce]: Pesachim 35b-36a
Liability for eating an improperly slaughtered animal on Yom Kippur: Pesachim 36a; Keritot 23a
Liability for an oath stating that one will not perform an act which is forbidden under the Torah's laws, as well as other acts: Makkot 22a
Liability for cooking and eating gid hanasheh [sciatic neurovascular bundle] in milk on Yom Tov: Beitzah 12a-b; Makkot 22a-b
Eating disqualified offerings while impure, on Yom Kippur: Keritot 13b-14a
Engaging in adultery and incest with a married relative who is also a niddah: Keritot 14a-b
Liability for multiple prohibitions in eating chelev [forbidden fat] from notar [a korban which has outlasted the timeframe for its use]: Keritot 23a-b
Is it possible to be liable for eating notar [leftover korbanot] of high-level offerings, given that one is already liable for meilah when he eats them, and we say ein issur chal al issur [an item prohibited in one way cannot gain a second layer of prohibition]?: Keritot 23a-b
Is it possible to be liable for eating pigul of high-level offerings, given that one is already liable for meilah when he eats them, and we say ein issur chal al issur [an item prohibited in one way cannot gain a second layer of prohibition]?: Keritot 23b
An exception: Issur kollel
Definition of Issur Kollel, per Rashi Keritot 14a: A second prohibition may apply for me if it includes a broader scope of items than the first prohibition did.
Example: I could not eat forbidden fat from an offering, because it was forbidden fat. I became impure, and therefore I could not eat any pieces of offerings, including this fat, due to this second prohibition: Keritot 14a
Example: I could not eat from a korban, because I was impure. Yom Kippur began, and therefore I could not eat anything, including this korban, due to this second prohibition: Keritot 14a
An exception: Issur mosif
Definition of Issur Mosif, per Rashi Keritot 14a: A second prohibition may apply for me if it includes a broader scope of uses of this item than the first prohibition did.
The new prohibition which applies to others need not be the same prohibition which would apply to me, in order for the exception of "issur mosif" to be employed - as seen in the case of one's daughter who marries one's father's brother. She becomes prohibited to his father's brother's brothers as a brother's wife, and therefore she adds a prohibition for her father as his father's brother's wife: Keritot 14a-b
Example: I could not eat forbidden fat, because it was forbidden fat. It was then dedicated to the Beit haMikdash, and therefore I could not benefit from it eather, due to this second prohibition: Keritot 14a
Example: I could not use forbidden fat from a korban. It then became illegal leftovers from that korban, and therefore the Beit haMikdash could not use it, either: Keritot 14a
Example: One's daughter married one's brother. She became prohibited as a brother's wife to his brothers, and therefore she became prohibited to him as a brother's wife [as well as his daughter]: Keritot 14a-b
Example: One's daughter married one's father's brother. She became prohibited as a brother's wife to his brothers, and therefore she became prohibited to him as a father's brothers' wife [as well as his daughter]: Keritot 14a-b
Example: One's daughter married someone. She became prohibited as a married woman to the world, and therefore she became prohibited to her father as a married woman [as well as his daughter]: Keritot 14a-b
Example: One's daughter became a niddah. She became prohibited as a niddah to her husband, and therefore she became prohibited to her father as a niddah [as well as his daughter]: Keritot 14a-b
Example: One's wife's mother married someone. She became prohibited as a married woman to the world, and therefore she became prohibited to her son-in-law as a married woman [as well as his wife's mother]: Keritot 14b
Example: One's granddaughter married his father. She became prohibited as a father's wife to her grandfather's brother, and therefore she became prohibited to her grandfather as a father's wife [as well as his granddaughter]: Keritot 14b