The SessionsCollectionRS class is responsible for accessing the config.sessions collection on replica sets. The _makePrimaryConnection() method is used to retrieve a connection to the primary node before performing some kind of operation on the sessions collection. 

Today, the implementation of this method retrieves a ScopedDbCollection, which comes from the internal connection pool, and then authenticates as the __system user on the connection before performing the requisite operation. After the operation is completed, the connection is returned to the pool.

As a result, it is possible that the connection from the pool has already been authenticated as _system. Ever since SERVER-56267, the auth subsystem now logs a warning whenever a client reauthenticates as the same user already authenticated on the connection. This can result in unnecessary log spam, so this method should strive to only authenticate as _system once.