The brain will always be more sensitive to jolts and subsequent concussions will more likely cause post concussion symptoms - but second impact syndrome is a separate thing that it is thought can only happen very soon after the concussion and which can kill you.

Me, I will never go back to contact sports, and I wish someone had persuaded me to stop doing them long before I got as many concussions as I did. So in turn I'm trying to persuade people on this thread, if you're already having enough concussion problems to be on this website, it aint worth it anymore, find another hobby.