Thank you for your replay.
I checked my activity carefully again and it looks like two more flows have triggered double notifications in the last 7 days.
I was also testing the same flows (recipes) with IFTTT. It is similar automated workflow solution to MS Flow (as you may know). And IFTTT has no problems with it. It always generates one push notification.
So definitely there is something wrong with checking RSS by Flow or with an iOS Flow app.
Maybe it is something with intervals between RSS checking.