According to Written in the Stars, Luke and Lorelai had known each other for eight years before they started dating. (They were discussing the first day they met, and he reveals that he's kept the "horoscope" in his wallet since that day, eight years before.)
Luke dated Rachel five or six years before the first season. She came back during the first season, and left again. Lorelai didn't know about Rachel when Rachel and Luke were dating, because she had presumably just met him, and had an 11 year old kid to worry about. (This is all explained in Concert Interruptus)