Play multiple sports bc it is fun. Will playing lacrosse year round make you better yes, enough to get you on the roster at Duke probably not. Chances are if you are talented enough to play for Duke working on your stick sticks year round and playing in the spring and summer while playing something in the fall and winter would be enough.
I see too may one sport kids that are average in their chosen sport but would help out teams in other sports.
Your athletic career is so short and only a chapter in your life. Play everything hard and enjoy it while it lasts. If you have the athletic talent they will find you. If you want to play something year round because you love doing it...fine. If you are doing it with the thought that it is going to get you that spot at Duke.... YOu better have the talent to back it up or you are wasting some great opportunities in your life that you WILL NEVER BE ABLE TO GET BACK! | 
