Unison notes of differing duration or dot-count across two voices must NOT be allowed to render at the same position

When two voices sharing a stave sound the identical pitch (true unison) but with different note durations or different dotting, the two noteheads must be kept visually distinct (offset), never stacked directly on top of each other.

A quarter note and a half note at the identical unison pitch, sharing a stave, rendered at the identical (x, y) position — the open (half) notehead and filled (quarter) notehead overlap and one becomes illegible, or the dot of a dotted note collides with the other voice's stem.

Gould, Behind Bars — surveyed practice varies on shared vs. separate unison noteheads generally, but is unanimous that unison notes of DIFFERING rhythmic value (or differing dot count) must be written with separate, distinguishable noteheads.

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