I have been to two conferences within the last 3 years or so offered by my employer. I almost went to the one this year, as Dr. Anderson (the foot guy) was supposed to give a talk about the ankle/foot. That was canceled so I did not attend. One conference was a general sports medicine conference, where the Seattle surgeon spoke about ACLs. The other was a concussion conference where they had Dr. Kevin Guskiewicz speak. He has a lot of memberships, but most notably he's a member of the NFL Head, Neck, and Spine Committee. He was one of the individuals who formulated the most recent concussion protocol. So many studies referenced, and he even referenced the NFL protocol, which apparently is proprietary and he could only give us limited information on it. One of the biggest things I took away from his talk was exactly what I said above. He specifically stressed tackling and taking a hit, and how certain athletes' style of play is more indicative of concussion risk. I remember being surprised, as I read my FBG articles too and even listen to the NFL commentators on game day... and I thought that once you get a concussion you're more susceptible to more concussions. It was surprising to hear that once the brain is healed, it's healed. Now that's a big if it heals. If it does not heal, then sure- another hit to the head can exacerbate those symptoms.
So take it for what it's worth. I know there are a select few here that think I blow smoke and I just make this stuff up as I go along. I heard it from his own mouth, so if you believe me great, if not that's fine. It is one opinion, and I'm sure there are studies out there that show one concussion leads to more, but I'd be curious if those same studies compare style of play as Dr. Guskiewicz specifically discussed; maybe it's not the athlete's previous concussion that has lead to more, rather maybe it's every time that player approaches contact they lower their head down and lead with that. FWIW Dr. G also said that there isn't a single helmet that can prevent the next concussion. He quoted Dave Halstead, from the Southern Impact Research Center, "If a player is suffering repeated concussions, the helmet selected should look good on the golf course, because that player should consider changing sports."