Each time I edit a note, another marker (for the same note) appears in the text to which the note is attached. The attached screenshot shows the result of editing a note:

Each time after editing the note I end up with another note marker:

The extra markers seem to be persistent - turning off notes highlighting and then back on again still shows 7 notes markers for the same note. So far the only solution I've found is to delete the note entirely and then recreate it (taking care not to edit it again... :-( )