As I am reading Leviticus, it makes sense that it would echo ideas given in Exodus, which was where our first reading come from. In this case, Leviticus 19:34 is a slight expansion on Exodus 20:20. Both sections tell us to treat foreigners (aliens) fairly as the Israelites were once foreigners in the land of Egypt. While the first reading just asks us not to molest or oppress foreigners, the reading in Leviticus tells us to treat them as we would those native born.
That theme is an expansion of Leviticus 19:18 which tells us to "... love your neighbor as yourself. ..." This is the commandment to which Jesus specifically refers in today's Gospel from Matthew. When the Pharisees challenge Jesus by asking which commandment is the greatest, Jesus gives them two. The first encompasses loving God with all our heart, all our soul, and all our mind. The second is directly a reference to the verse in Leviticus. Together, these two commandments encapsulate all of the commandments and laws given to the early Israelites.
Today, I was reminded not just of how recurring themes crop up in the Bible, but in my life as well. God knew that I needed these reminders today as I have been struggling with the "love your neighbor" aspect lately. Even foreigners, in both birth and in mindset, deserve my love and respectful treatment. The Lord knows that He needs to repeat ideas to me to get them through sometimes.