could indeed be a contraction of Hugobert that later came to be used as the normal form (remember, there are over 1,500 years in which names like Hugobert were common; that's a lot of time for evolution, needless to say). Hubert
could've originally been a nn, but I think it's rather impossible to tell.
But I actually think that it's rather more likely that Hugobert was a rare "learned" lengthening
, since my reliable sources (BtN, A Dictionary of First Names
by the Oxford Press, Oxygen.com's Babynamer) don't mention Hugobert in their entries about Hubert
Proud adopter of 15 punctuation marks; see my profile for their names.