[Mesorah] suffix for a family: "uni" vs. "oni"

[Kenneth Miller]

R' Seth Mandell wrote: <<< The plain answer is that Gershon is really Gershunn, with a double nun, which disappears at the end of the word.  When a vowel is added to the end, the doubling comes back, and so it is Gershunni.  This not like Z’vulun, where the nun is not hiding any doubling. >>>

Why would the doubling change an "o" into an "u"?

Akiva Miller
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