Back when Duerson played (and for pretty much Seau's entire career), players would just get right back on the field without getting properly assessed. What's sad is that even though they've improved on this, you still have situations like Colt McCoy's last year. He was obviously concussed but they kept him on the field.
The brain is MUCH more susceptible to concussion after you've already suffered one--and playing through a concussion is basically a guaranteed ticket to some form of brain damage down the line.