Two voices sounding the same pitch but spelled with different letter names at the same onset can read as a printing error

When two voices sharing a stave sound the same actual pitch but are spelled differently (e.g. F#4 in one voice, Gb4 in the other) at the same instant, a reader can mistake the mismatch for a copying error rather than a deliberate enharmonic choice, unless the harmonic context makes the differing spelling clearly purposeful.

F#4 in the Soprano and Gb4 in the Alto at the identical onset, in a passage with no clear harmonic-function reason for the respelling.

General engraving practice (Gould, Read) on maintaining spelling consistency across simultaneous voices absent a specific functional reason to differ.

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