A second copy of the same thing. Duplicates inflate a count and are the commonest way an archive lies about its size.
Latin duplicare, to double, from duplex (twofold), from duo (two) plus plicare (to fold).
Its root is a fold, not a copy: duplex named a thing folded into two layers, one laid exactly over the other. A duplicate is the record equivalent of that fold, the same layer counted twice because nobody smoothed it back flat.