No evidence that concussions reduce brain's physical tolerance
 

There's no evidence that getting a concussion means that in the future, you will get another concussion from less acceleration of the head as a result of sudden forces applied to the skull.

The source reads:

Source: http://www.ns.umich.edu/new/multimed...young-athletes

There have been various studies about second/subsequent hits and such with varied results. It is very difficult to adequately categorize those second hits or the force of the multiple little hits.

People still need to try to avoid hitting their heads.