verb/ɑ̃.fɔ̃.se de pɔʁ.t‿u.vɛʁt/
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
Related: enfonceur de portes ouvertes, enfonceuse de portes ouvertes
Literally, “to push in open doors”. Compare Italian sfondare una porta aperta, German offene Türen einrennen, Russian ломи́ться в откры́тую дверь (lomítʹsja v otkrýtuju dverʹ), and the same in most other European languages.