Two identical pitches in one chord must share a single notehead, not render as two coincident noteheads

When two voice parts land on the exact same pitch at the exact same onset within what Gradus encodes as a single chord array (e.g. `pitch: ['C4','C4']`), standard engraving draws ONE notehead, not two stacked copies — a duplicated pitch in a chord array is an authoring error, not a legitimate unison-in-chord notation (real cross-voice unison is a DIFFERENT case, see vs-cross-voice-unison-collision, which uses two independent voices, not one chord array).

`notes: [{ pitch: ['C4','C4'], dur: 'q' }]` is authored (accidentally, e.g. from a copy-paste), and VexFlow's StaveNote receives two identical `keys` entries, rendering two exactly-coincident noteheads that read as a single (correct-looking) note but silently double every downstream index (accidental/articulation targeting `idx` 1 hits a phantom duplicate).

Gould, Behind Bars, ch. 'Note relationships' — unison notation; general convention across all engraving software that a chord array is deduplicated by pitch before layout.

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