1. (不必要的)重复
If you say that there has been duplication of something, you mean that someone has done a task unnecessarily because it has already been done before.

1. the act of copying or making a duplicate (or duplicates) of something

e.g. this kind of duplication is wasteful

Synonym: gemination

2. a copy that corresponds to an original exactly

e.g. he made a duplicate for the files

Synonym: duplicate