This is not a mere "vain repetition" of Exodus 12:15. It adds an important extension of the punitive clause—"that soul shall be cut off from Israel"—from Israelites proper to proselytes. We are thus reminded, at the very time when Israel is about to become a nation and to enter upon its inheritance of exclusive privileges, that no exclusion of the Gentries by reason of race or descent was ever contemplated by God, either at the giving of the law, or at any other time.
In Abraham all the families of them were to be blessed (Genesis 12:3). It was always open to any Gentiles to join themselves to Israel by becoming "proselytes of justice," adopting circumcision and the general observance of the law, and joining the Israelite community.
The whole law is full of references to persons of this class (Exodus 20:10; Exodus 23:12; Le Exodus 16:29; Exodus 17:10; Exodus 18:26; Exodus 20:2; Exodus 24:16; Numbers 35:15; Deuteronomy 5:14; Deuteronomy 16:11-14; Deuteronomy 24:17, Deuteronomy 24:19; Deuteronomy 27:19; Deuteronomy 29:11, etc.
). It must have been largely recruited in the times immediately following the exodus from the "mixed multitude" which accompanied the Israelites out of Egypt (Exodus 12:38), and from the Kenites who joined them in the wilderness (Numbers 10:29-31; 1:16).
Born in the land—i.e; an Israelite by birth—"the land" is, no doubt, Canaan, which is regarded as the true "Land of Israel" from the time when it was assigned by God to the posterity of Abraham (Genesis 15:18).