What's a double concussion?

Very dangerous: There's a syndrome called "second impact syndrome" which can occur as someone has a second concussion while recovering from their first concussion. The resultant injury to the brain can cause malignant cerebral edema (swelling) and this can be fatal- mortality is 50-100%.

Second Impact: The reason concussion protocols for "return to work or sport" should be followed so exactly is because of this dangerous syndrome. Basically, an individual has a second concussion while the brain is trying to heal from the first concussion. Perhaps they returned to activity too soon or did not follow protocols well. The second impact is often fatal or permanently damaging.