In Polish, and I believe in a number of other European languages, there is an idiomatic expression which translates to "to force a door which is already open". It is used to describe a situation when much effort is spent on solving a problem which has already been solved, or did not exist in the first place. For example, if someone was doing research to answer a question which has already been answered, one would be said to be "forcing an open door". The same would apply to a person who is arguing a point that everyone already agrees with.
As far as I am aware, this phrase does not carry similar meaning in English. (Or does it?). Hence, I would like to know if there is an idiom with similar meaning? If not, is there a succinct way to describe such situations?