No redundant restatement of an unchanged dynamic

A dynamic mark is only written where the level changes (or after a structural break like a rest of significant length or a new system/movement); repeating the identical mark on consecutive notes with nothing intervening is visual clutter.

Two consecutive notes in the same voice both carry `dynamic: "mf"` with no dynamic change, hairpin, or new phrase between them — the engraving shows "mf" twice within a couple of beats.

Gould, Behind Bars, ch. Dynamics — "do not repeat a dynamic unless there has been an intervening dynamic change"; Ross concurs, noting dynamics are stated once and persist until contradicted.

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