The opening of this verse repeats verse 12b for rhetorical emphasis and with only slight modification.

So shall my covenant be in your flesh an everlasting covenant: So marks a conclusion such as would “Thus,” “The result of this is that….” The idea of a covenant being in your [plural] flesh may seem strange. The meaning is that circumcision will show, be evidence, serve as a sign, that God’s covenant will last forever. We may translate, for example, “By doing this your bodies will show that my covenant [promise] with you will last forever,” or more formally, “In this way my covenant with you will be marked [engraved, stamped] in your flesh to show that it is an everlasting covenant” or “… that I have made a covenant with you that will last forever.” One translation expresses it this way: “This mark will be the proof in your bodies that my promise with you is for good [for ever].”

Quoted with permission from Reyburn, William D. and Fry, Euan McG. A Handbook on Genesis. (UBS Helps for Translators). New York: UBS, 1997. For this and other handbooks for translators see here .