I always understood that if a person does something that warrants capital punishment, and there are witnesses who warned them against the crime, that's all they are punished with. If it was done intentionally but without witnesses or a warning, for certain crimes they get punished instead with kares. But I wouldn't think that the person who is getting the capital punishment also gets kares.
The Rambam in Sefer HaMitzvos in a few places sounds like this. For example, at the end of Mitzvah Lo Sa'aseh #336 he writes:
הנה התבאר מכל מה שקדם שהעובר על לאו בתו ובת בנו ובת בתו הוא בשריפה, ואם לא נחקרה העדות בכרת אם היה מזיד, ואם היה שוגג באחת מהן יקריב חטאת קבועה
Behold, from all the above it has been explained on the prohibition of a person with their daughter, son's daughter, or daughter's daughter that the punishment is death by fire. If there wasn't sufficient testimony then the person gets kares if it was intentional, and if it was unintentional they bring a chatas offering.
However, in the next mitzvah, #337, he writes differently:
חייב שריפה אם נחקרה העדות, הוא בכרת גם כן אם היה מזיד. ואם היה שוגג יקריב חטאת קבועה
They're punished with death by fire if there were witnesses. If they were intentional they also get kares, and if unintentional they bring a chatas offering.
Am I misunderstanding the Rambam? Was my whole premise mistaken?
The above quote from #337 is from the Frankel Edition. However, the version on Sefaria (which they say is the Warsaw 1883 edition) reads differently, in a way that there's no issue:
חייב שריפה. ואם לא נחקרה העדות בכרת אם היה מזיד. ואם היה שוגג יקריב חטאת קבועה
They're punished with death by fire. If there weren't sufficient testimony, they get kares if it was intentional, and if unintentional they must bring a chatas offering.
I decided to check the Rav Kapach edition, and his is like the Frankel edition. The original Arabic reads:
ילזמה שריפה אן אסתקלת אלשהאדה, והו בכרת איצ/א אן כאן מזיד, ואן כאן שוגג יקרב חטאת קבועה.
My Judaeo-Arabic is pretty rusty, but judging from other mitzvos in this sefer, the word
גם, or "and".
Something relevant is the following foonote (#70) on the word גם in Rav Kapach's edition:
במהדורתי הקודמת הושמטה תיבת "גם" כי לא הובנה
In the previous edition of my work, the word "also" was removed, since it's incomprehensible.
What this shows is Rav Kapach is also bothered by this question. I don't know why he added back the word "also" if he doesn't understand it. I guess he wanted to be faithful to the original text. Does anyone explain this?
Edit: I just saw the Rambam at the end of #352 says explicitly my premise:
וכן התבאר לך שכל מי שנתחייב מיתה בבית דין איזו מיתה שתהיה הנה האיש ההוא בכרת גם כן כשלא המיתוהו בית דין או לא נודע לו
It has also been explained to you that anyone who is liable to the death penalty, whichever death it is, they also are subject to kares when they aren't put to death, or their crime wasn't known.