WordNet records no synonym and no antonym for double flat. It is the only member of its synset, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
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.
a musical notation of two flats in front of a note indicating that it is to be lowered by two semitones
Synonyms, antonyms and the broader and narrower relations for double flat come from Princeton University’s WordNet. Full sources and licences.