Etymology of phrases 靜雞雞 and 靜靜雞. Are both correct, and is there any difference in meaning?
Both 靜雞雞 and 靜靜雞 means 'quietly' and/or 'stealthily'
The minor difference are on the emphasis

靜雞雞 emphasize on the 'stealthily' part
靜雞雞喺外面收埋個二奶 = &quot;quietly and stealthily hiding a mistress outside&quot; (Since the main point is the secrecy, the 'stealthily' aspect is more prominent than the 'quietly' aspect)

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靜靜雞 emphasize on the 'quietly' part
e.g. 靜靜雞企埋一邊 = &quot;quietly and stealthily step aside&quot; (Since everyone can see you step aside, it is not very stealthily, therefore, the 'quietly' aspect is more prominent than the 'stealthily' aspect

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靜靜雞喺外面收埋個二奶 would emphasize on the 'keep quite' aspect
靜雞雞企埋一邊 would emphasize on the 'inconspicuous manner' aspect


靜雞雞 is listed in the Cantonese dictionary CantoDict

靜靜雞 is not listed. It seems to be a variant of 靜雞雞 by popularity (enough people say 靜靜雞  instead of 靜雞雞 made it a legit phrase)


Edit:
Base on some Cantonese coined unique reduplicated adjective, I think 靜雞雞 is the standard one
儍更更 (adj)
烏啄啄 (adj)
懵盛盛 (adj)
黑麻麻 (adj)
白雪雪 (adj)
The common form of reduplication that both Cantonese and Mandarin use:
靜 (adj) --&gt; 靜靜 (adj); 靜靜噉/ 靜靜地 (adv)
好 (adj) --&gt; 好好 (adj); 好好噉/ 好好地 (adv)

I believe 靜靜雞 came from mixing of 靜靜噉 and 靜雞雞