Reish is originally on of the בג"ד כפ"ת words (see Sefer Hayetzira).
A dagesh kal is a phonetic anomaly in where it is more comfortable to pronounce the letter as raphe (not hard), as apposed to mudgash kal (lightly stengthened).
Our modern day "dh", "th", "gh", "ph" etc. stem from this as proven in R' Eliyahu Bachur's book that is translated to latin where the equvalints of the scripted beged kefet is combinations of usuaul letters with an h (with the exception of b, as the pronounciation of Bet rafe or dagush seems to be indistinguishable in latin).
This is caused by an unclosed vowel in a word moving on to the beged kefet letter.
But when starting a vowel with a beged kefet letter it will be mudgash.
The same priciple can be demonstrated with 'L', where in the beginning of the vowel it will sound hard, and in the middle of a vowel it will be softened.
The abovementioned priciples apply to Riesh (but was forgotten by most throughout the generations) where at the beginning of vowel it will be rolled twice, in the middle-once.
This is backed by the hebrew encyclopedia, and by a Yeminite who knows dikduk backwards.