quadruplicate has 4 distinct synonyms spread across 1 of its 3 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
any four copies; any of four things that correspond to one another exactly
“it was signed in quadruplicate”
reproduce fourfold
“quadruplicate the bill”
having four units or components
“quadruple rhythm has four beats per measure”
Synonyms, antonyms and the broader and narrower relations for quadruplicate come from Princeton University’s WordNet. Full sources and licences.