double effort; doing the same work twice
Refers to wasted effort caused by having to redo something or do it again unnecessarily.
Rewriting the documents is double effort.
If I had checked first, it wouldn't have been double work.
手間 means effort or time spent on a task; 二度手間 specifically means that effort is doubled because the task must be done again.
Compound of 二度 (twice) and 手間 (effort, time).