Registering adapters with the same ID no longer raises, but does not add...

This allows an adapter to be registered again without raising an error - beyond the first, subsequent attempts are silently discarded. There doesn't seem to be any value in raising the error beyond ensuring that the @adapters array contains only unique adapters. I noticed this when, in a setup with a master/slave postgresql pair, Makara seemed to try to register every time an ActiveRecord connection was made.