Two voices coinciding in pitch and duration on a shared stave should share one notehead (or Gradus should deliberately choose the double-notehead alternative)

When two voices sharing a stave land on identical pitch for identical duration at the identical position, the traditional engraved convention is a single notehead with stems drawn in both directions, not two independent overlapping noteheads.

Two voices sharing a stave (SATB or two-voice stacked layout) with identical pitch+duration at the same slice index rendered as two overlapping StaveNote objects.

Gould, Behind Bars, Divisi and unaccompanied unisons (~, single-notehead-double-stem convention, but Gould also explicitly notes the double-notehead alternative is now common and arguably clearer); Ross gives the same single-notehead guidance for chorale-style unisons.

This rule needs the notated content and the rendered result together — the score says what should happen, the page says whether it did.