For lots of nicknames, first a name gets shortened, like Elizabeth to Liz, and then it gets lengthened again to Lizzy.
Dickon/Diccon is a lengthened form of Dick, the nickname for Richard, and was also used of, though probably not by, KingRichard III. I knew a man whose full given name was Dickon; never knew why, though. Perhaps his parents just liked it, or perhaps there were abundant Richards in the family and they wanted to keep the tradition but with their own slant on it.
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