What you want to do is globally match a something including possible repetitions, and replace what you've found with that something followed by the length of your match:

— Arien

Edit: It seems I misread the output you want. To only have sequences of two or more repeated letters replaced, change the star to a plus sign. (And after some sleep...) Also, you'd want to swap length $1 and $2 to have the length preceed the letter.