A second MCAT score that is lower than the first may be concerning to some schools, but it's hard to say. I'm not in a great position to comment, but from my experiences (in talking with admin, friends who applied to schools, etc), it's not a massive red flag. Moreso, I would be wondering why you would retake it considering a 516 is a really good score (unless you're solely focused on T20 schools). The more MCAT attempts you take, the more it hurts your app (I say this looking from a lower perspective). If you can guarantee yourself a 520+, then it may be worthwhile, but you also run the risk of only increasing by a few points. You're at the MCAT level where it's only a few questions that can differentiate you from a 516 to a 520+. A good thing I've noticed is more schools are starting to look at applications holistically, factoring in other variables on top of multiple MCAT scores. I don't know what T20 schools look for exactly (and how they would view a decrease), but I would be hesitant on a third attempt and really look at different weak points in your app (increasing MEANINGFUL nonclinical volunteering, research opportunities, how you're wording your primaries/secondaries, etc). Sometimes that's the difference.
But take this with a grain of salt. By no means am I in your position. I did not do as good as you on the MCAT and have been really focusing on other parts of my app, but not in the best place to comment. This is my opinion and hopefully it can help provide you some insight + thoughts on how you would like to proceed. Feel free to message me with questions.
