Rashi on Deuteronomy
נכון [AND, BEHOLD, IT BE TRUE, AND THE THING BE] CERTAIN — i.e. the evidence of the witnesses is in agreement (מְכֻוָּן)
Tur HaArokh
והוגד לך ושמעת, “and you have been informed, and you will hear, etc.” The apparent repetition of the same thought means that whenever and wherever this accusation is brought to your attention you must immediately commence an investigation. As soon as you establish that the rumours you have heard are factual, you have to lead the guilty party to his or her place of execution where you will stone the guilty party to death.
Rabbeinu Bahya
ודרשת היטב, “you shall investigate thoroughly;” the numerical value of the word היטב equals the numerical value of the letters in the tetragrammaton, i.e. 26. It is as if the Torah had written “you shall investigate him in your capacity as the agent of Hashem.”