The underlying Germanic form was reuk-a, according to Kluge's etymological dictionary. Then, in the beginning of the High German consonant shift, which set apart Old High German from the other Germanic languages, voiceless plosives /p t k/ regularly became geminated fricatives /ff ss xx/ in intervocalic position, which we have in reuka. The <hh> is only a spelling of this /xx/. (In some other positions, the plosives turned into the affricates /pf ts kx/.)
Parallel examples of the same phenomenon are
makon > mahhôn "make"
reikeis (actually got.) > rîhhi "rich"
The existence of the geminated fricatives is nothing unusual, they appearently had existed before in Germanic (in contrast to the affricates), according to my reference (Wilhelm Schmidt: "Geschichte der deutschen Sprache", 2007). The shift was a long process, though, and the concrete form /riohhan/ is only one snapshot out of some centuries.
Now, why the consonant shift occured is difficult to say and still researched. It probably started in some corner of the language and then proceeded by chain shift effects.
It is not geminated. The word contains two separate h's in to separate syllables. Geminated consonants exist only within the same syllable.
The first h is a length marker that will not be pronounced. It still exists in modern standard German at the end of some syllables and is called Dehnungs-h:
Floh, früh, Vieh, ...
The second h is the first consonant of the next syllable. Also in modern standard German it can happen, that a syllable, that ends with a Dehnungs-h meets a syllable that begins with the consonant h:
Strohhut, Schuhhaus, Gehhilfe, Kuhhandel, Frühherbst, Viehhandel, ...