A SUSTAINED passage where two voices move in identical rhythm and pitch is marked 'a2' / 'unis.' and reduced toward a single notated line, not repeated as two coincidentally-matching voices bar after bar

The per-onset opposed-stem unison rule above governs an incidental unison — two voices happen to land on the same note for a beat or two. A SUSTAINED passage (a full phrase or more) where two voices move together in lockstep pitch and rhythm is a different situation: standard orchestral/choral practice marks the passage 'a2' (or 'unis.' in choral writing) above the stave and, in many house styles, drops to a single notated line with the marking rather than continuing to duplicate every note in two voices for the passage's duration.

An 8-bar passage where voice 1 and voice 2 are pitch-and-rhythm identical on every single note, still fully duplicated as two independently-stemmed lines with no 'a2'/'unis.' marking and no reduction — technically correct but needlessly cluttered, and a missed opportunity to signal the compositional fact (these parts are doubling) to the reader.

Standard orchestral engraving convention for divisi/unison markings (Gould, Behind Bars, ch. on divisi and unison markings; Ross, The Art of Music Engraving, describes 'a2'/'unis.' practice directly). Distinct from the incidental-unison rules above, which Gould treats as a per-note notehead question rather than a passage-level reduction.

This rule can be decided from pitches, durations and voices — no layout required. A validator can check it before anything is drawn.