What do we learn from the double expression, "Himol Yimol"?
Chizkuni: The two stages of Milah and Peri'ah. 1
Targum Yonasan, Ohr Ha'Chayim (to 17:23): We learn from here that only someone who has himself been circumcised is eligible to perform the Bris Milah. 2
QUESTIONS ON RASHI
Rashi writes: "This teaches that in one case, a servant born to the household is circumcised at eight days; [while in a different case, he would not be - for this verse does not mention the eighth day]." How can we determine which type is which?
Gur Aryeh: Logic dictates that If one purchased a maidservant and she gave birth while in his household, that child would be circumcised at eight days, for the mother is a member of the household as well. If one purchased the rights to the offspring without purchasing the mother, that child would be circumcised even at one day.