The wiredTiger storage engine is registered twice with the global environment. This is to accommodate passing both "wiredtiger" and "wiredTiger" as engine name at the mongod command line.
SERVER-16266 we added "wiredTiger" to make the naming consistent and left "wiredtiger" as we had released an RC. I would argue however that we should remove "wiredtiger" now as there will be a larger impact if removed in the future.
Outside of simplifying the command line, removal of "wiredtiger" comes into play when iterating over the list of storage engine factories via the StorageFactoriesIterator. The Factory interface provides engine identification via the getCanonicalName() method. This method will return the same name, "wiredTiger", for both registered WiredTiger instances. De-duping is then required if looking for a list of registered storage engines.