In the Avotaynu book of surnames, there are the names : Rafel, Rapel, Rappel.
Here's a thought ...
If the surname were to be written in hebrew letters, the sounds of 'f' and 'p' are represented by the same letter fey/pey. The way to distinguish the sound of the letter under discussion in Hebrew is to place a dot inside the letter and then we know the sound is pey so it would be Rapel.
However, if one is unsure of the pronounciation then over time if the name is written without a dot and thereafter in latin letters it could become either or both of Rafel/Rapel.
I have come across this issue of hebrew pey/feh in connection with the first letter of my surname.