Idiom: To bring sand to the beach
Sandra and Jane are discussing a friend's up-coming birthday...
Sandra: I don't know what to get Julia - and her birthday party is this weekend!
Jane: I know she likes bracelets. She certainly has a lot of them.
Sandra: No kidding! She has so many that if I give her another, it'll be like bringing sand to the beach!
Meaning: Because there is so much sand at the beach, there is no point or purpose in bringing sand. When to bring sand to the beach is used, it means the action a speaker suggested or choice a speaker has made (in the above example, buying yet another bracelet for a person who has many) is without purpose or good reason.