to carry coals to Newcastle; to bring owls to Athens (do some redundant and pointless)

(idiomatic) to sweat bullets

(idiomatic) to jump in at the deep end

(idiomatic) To preach water and drink wine; to fail to practice what one preaches.

blood is thicker than water

(colloquial) to put one's pants on one leg at a time (indicating that others are not performing better, are not superior, are nothing special)

in one's element; at home / as it should be; perfectly

(idiomatic) to be grist for someone's mill

(idiomatic) to not hold a candle to

(idiomatic) as sound as a bell