I see some old posts about the notifications app not supporting repeating notifications within a day but then some that contradict that. And the Only once within this number of minutes option makes it seem possible. I thought this used to work for me but noticed recently it's not.
Ah, thanks for the screenshot Bruce - that helps a lot! For some reason I totally missed the "repeat notifications" control and if you enter a number in the "Only once within this number of minutes" box then that control goes away (and it didn't occur to me to clear the number).