Comment by kazinator

This is about idempotent operations, which are basically state changes that have the same affect if executed multiple times as if executed once:

  $ chmod a+x file
  $ chmod a+x file

This is linked to mathematical idempotence in the sense that an operation is a function which takes some inputs i and the state of the world S and produces a new state S':

S' = f(S, i).

So then if f(f(S, i), i) = f(S, i) then f is idemponent mathematically, and the operation is idempotent in the software sense.