When there are two notes of the same pitch in opposing voices (stem-up and stem-down) at the same time-point, Sibelius overlays the noteheads. This is incorrect when the notes are of differing lengths (e.g. one has a rhythm dot and the other does not): in this case the notes should be offset to avoid overlaying each other. The noteheads should overlay only when they are of identical lengths.
Idea No. 65