Here is from
http://p211.ezboard.com/fwordoriginsorgfrm1.showMessage?topicID=14087.topic:
"It rhymes. (Well, the roots do.)
Moreover, "duck" has a substantial history of being used to refer to
persons: as a term of endearment as far back as Shakespeare's time, and as
a slang term equivalent to "fellow", "chap", or "guy", back to the mid-19th
century. "Lucky duck" isn't listed in the OED, so I don't have any evidence
as to first use (without investing more time than I'm willing to in
searching old publications) but it seems to me that it's just an example of
the latter slang use, probably persisting because the rhyme makes it
euphonious, as DW points out. There's also the expression "odd duck",
meaning a strange or eccentric person, which seems to be another
manifestation of the same sense of "duck" = person."
