It means both copies should be identical, with no variations between them.

Exact Copies

'In duplicate' is often used in official or formal paperwork requirements.

Paperwork Protocol

Creating documents in duplicate is common in legal contexts for record-keeping.

Doing something 'in duplicate' in science ensures accuracy and reliability of results.

Repetition in procedures is meant to verify consistent outcomes.

Performing tasks 'in duplicate' can help catch and correct errors early.