You need to delete events from the Reminders calendar first, so we can create new ones and they won’t appear duplicated.

I wrote all the steps you need to create the shortcut in my previous reply, but I made an example shortcut which adds all your reminders due the next 30 days to a calendar named “Reminders” (you have add this calendar first).

https://www.icloud.com/shortcuts/b388d14043434ff995e32ec4c118d51e

Pay attention when you set up this shortcut, because if you set the wrong calendar name, its gonna remove all events from it when you run it.