Notion 6 (and previous versions) uses numberpad 2, 3, 4, 5, 6, 7, 8 to add a duplicate interval above (or, with control, below) the note.
Numberpad only seems to work on PC keyboards, not Macs (which almost but not quite uniformly don't have number pads).
Is there a modifier key which can be used to give access to the numberpad add interval functions?
The fact that you have to use the numeric keypad, or number-pad to the side of the main keyboard, is frustrating here, as there does not appear to be any alternate keystroke or menu item which allows duplicating e.g. an octave above.