neck and neck has 2 distinct synonyms spread across 2 of its 2 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.
inconclusive as to outcome; close or just even in a race or comparison or competition
“as they approached the finish line they were neck and neck”
even or close in a race or competition or comparison
“the horses ran neck and neck”
Synonyms, antonyms and the broader and narrower relations for neck and neck come from Princeton University’s WordNet. Full sources and licences.