enfoncer des portes ouvertes

Etymology
lit. 'to push in open doors'. Compare 🇨🇬, 🇨🇬, 🇨🇬, and the same in most other European languages.

Verb

 * 1)  to state the obvious as though it were new or controversial; to try to force or persuade someone who is already convinced; ≈ to preach to the choir