To remove repeated copies so each thing is counted once. Deduplication is how an archive stops lying about its size.

A second copy of the same thing. Duplicates inflate a count and are the commonest way an archive lies about its size.

Two things taken together as one unit. A pair is more than two — the pairing is itself a fact.

Sharing much without being the same. Similarity is always along some axis, and naming it is most of the claim.