The new grammar rule must have caused a bit of confusion in Korea because some Koreans in some regions, even today, mark the subject of a sentence by using 이 and 가 together, maybe just to be safe. Consider the following sentence:
당신 너 딸이가 찾아왔습니다.
Your daughter is here looking for you.
Notice how 이 and 가 are both used to mark the subject?
I have read that 네가 and 제가 are probably abbreviated forms of 너이가 and 저이가.