Comment 1 for bug 1595964

Technically one could argue that it is idempotent. Running it twice does not result in different things: build(build(x)) == build(x).

However, I see the complication you're encountering when trying to extend a plugin; I've run into the same issues myself. The reason the build directory is removed at the beginning of the build step is in the case of a previously failed build. We're working toward being able to resume such builds, but it requires some other things to happen first.