It was probably my post you were referring to. I wanted to be able to ride Tower of Terror twice in one day using FP+. For both of my HS days, I booked FP+ for RNRC, TOT, and Star Tours. I then immediately cancelled the FP+ for Star Tours on both days so now I only had 2 FP+ slots tied to my account.
Once I rode RNRC and TOT, I was able to book an additional FP+ at the kiosk and sure enough I could select from duplicates. One day I did RNRC again (which is Tier 1 so was able to ride TWO Tier 1 attractions!) since TOT was not available, and the 2nd day I was able to choose TOT from available attractions.
So it's a sneaky way of getting to ride a duplicate attraction or book duplicate tier ones, but I would advise only cancelling if you KNOW your plans are rock solid and will not be changing as it requires a phone call to Disney's IT in order to get that 3rd FP+ slot back! Also it's dependent upon crowd levels and fastpass availability - there's no guarantee that what you want to reride is even going to be available to book and therefore may not even show up as a choice...
Not sure why it needs to be same day. I cancelled my 3rd FP+ weeks before my trip and it worked out for me.