[unnecessary teaching] preach to the wise, teach one's grandmother to suck eggs, teach granny to suck eggs; preach to the converted.
"Teach your grandmother to suck eggs, Rhiannon
Moore! I learned at a master's feet; even I could not recognize
Holmes in one of his clever disguises unless he allowed it."
The phrase "teaching granny to suck eggs" would be completely impenetrable to most people who did not grow up in the UK (or a Commonwealth country.) It's almost as impenetrable as "Bob's your uncle!" (another UK phrase that every Brit knows and no-one else does.)
For the uninitiated, "Teaching your grandmaoher to suck eggs" means (approximately) trying to give advice to someone who is an expert and knows much more than you do.
http://davidmaister.com/blog/285/