Redis' master/slave replication is strictly one way, from the master to the slave(s). Any updates to the master are replicated downstream and overwrite the slave's contents.
If your replication is runnnng, all master updates will be replicated almost immediately to the slave. If you use writable slaves, keys that exist in both master and slave will be overwritten by the master's replication stream. New keys that exist only in the slave (when writable) will never be replicated upstream to the master. The only way to add keys to the master is by writing directly to it (or if it is itself a replica, then to its master...).
The question remaining is why would you want to write keys to the slave and have them shipped to the master as well. You could, theoretically, have your application write to both master and slave but I shiver thinking about the race conditions and consistency issues that you'll run into if you do that.
