Is there any difference in meaning between lev and levav? In the Shema a person is commanded to love "b'chol levavcha" with all his heart. But in Shemot 9:14 Hashem says he is sending plagues "el lib'cha" to your heart. Lib'cha (and libecha) and levavcha (and levavecha) both appear textually. Is there any exegetical distinction which would help me more fully understand why one is used in a specific location and not the other? I have found mention online of one rashi which sort of addresses this but not in any comprehensive way, so any guidance would be appreciated.
See Rashi's commentary. He says with both of your inclinations - the good and the bad.
Frequently a word that has a double letter such as this one implies a "doubling". See similar example in one of the Hallelukah - חרב פיפיות בידם - means a double edged sword. In this verse, view Siftei Chachamim commentary on what Rash"i says. He supports exactly that theory as well as answering exactly your question on the usage of the double bet here.