English: “You shall surely kill him” — this is already included in the derashos of the preceding verse.
On the daf / in context: The emphatic double language reinforces the duty of execution already derived from the prior commands of non-pity and non-concealment.
Type of Learning
Derash on the verse (Sanhedrin) — reinforcing the duty of execution already established.
What Is Learned
The doubled language “harog tahargenu” underscores the absolute duty to put the mesis to death.