Accidental duration is scoped to one bar, one letter name, one octave, one voice

An accidental governs only the exact written pitch (letter name + octave) in the measure in which it appears, for the remainder of that measure, in the voice/part it was written in. It does not apply to the same letter name in a different octave, to the enharmonic respelling of the same pitch class, or to a different voice sharing the stave.

A lesson bar states F#4 on beat 1, then a later F4 in the same bar with no accidental prints as F natural -- but the author actually intended F# (forgot to repeat) or vice versa.

Gould, Behind Bars, 'Accidentals' chapter (opening pages); Read, Music Notation, ch. 6; Ross, The Art of Music Engraving, ch. 5. All three agree on same-octave/same-bar/same-voice scope. (Exact page numbers in the first-pass draft could not be verified against a physical copy in this pass.

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