When to Use Duplicate and Duplicate?

is that duplicate is one that resembles or corresponds to another; an identical copy while duplication is the act of duplicating, or the state of being duplicated; a doubling; a folding over; a fold.

Is it correct to say duplicate copy?

Duplicate signifies that the copy is exact (not redacted). In this case duplicate copy is not redundant; it is a qualifier. Could be stated otherwise: Please sign and return the duplicate edition of the enclosed document. Unless you regard the envelope as containing two copies of the information.

When should you duplicate code?

Duplicate code makes the maintainability of a codebase suffer hard. One of the reasons duplicate code gets introduced to a codebase is because of poor design. The most effective way to prevent duplicate code from slipping into the codebase is by dividing code and logic into smaller reusable units.