Is there any phrase/expression to express the situation where someone is preaching or giving advice to someone who already understands and agrees with them and the whole speech is unnecessary. Something like "Barking up the wrong tree", but this expression conveys something different and not what I am looking for.
I found the expression "Preach to the choir", which exactly expresses what I would like to convey.
Fig. to make one's case primarily to one's supporters; to make one's case only to those people who are present or who are already friendly to the issues. idioms.thefreedictionary.com
The phrases persuading the persuaded (ELU,1), beating a dead horse (ELU,2), pushing at an open door (ELU,3) and carrying coals to Newcastle (ELU,4) also are sometimes used as alternatives to or substitutes for the phrase preaching to the choir. Also see Are "preaching to the choir" and "preaching to the converted" synonymous.
The German version of this meme is expressed as "Carrying owls to Athens" ;-) But I'm sorry, I don't really know if there exists a version of this in English. I just find the metaphor beautiful enough to share it.