There’s a reason Konstantinos Konstantinides never heard of this practice: it had dropped out of use in Modern Greek early in the 20th century. As in fact had the initial rough breathing on rho.
The ῤῥ orthography used to be regular in Western typography, but has long since fallen out of use; from memory, it was routine in early 19th century editions of Classical texts, and rare by late 19th century editions.
The ῤῥ orthography reflects a phonological reality of Classical Greek, that the second rho in a pair was voiceless, something attested in Herodian. Allen’s Vox Graeca (p. 39), who mentions the evidence, also refers to writing ῤῥ as a Byzantine practice, and it is of course corroborated in the Latin transliteration <rrh> (e.g. Pyrrhus = Πύῤῥος).