tl;dr：不要亂來啦... 這是 UB (Undefined behavior) 的一種。
因為看到這則 tweet，所以去查一下 JSON 的資料：
Google 教你如何往 JSON 里面写注释 pic.twitter.com/pDL6U6jtTa
— 魔法少女 ☆ poi 酱 (@PoiScript) November 27, 2017
Douglas Crockford originally specified the JSON format in the early 2000s; two competing standards, RFC 7159 and ECMA-404, defined it in 2013. The ECMA standard describes only the allowed syntax, whereas the RFC covers some security and interoperability considerations.
ECMA-404 裡面就真的只講語法沒講其他東西，而在 RFC 7159 內的 Object 則是有提到 (重點我就用粗體標起來了)：
An object structure is represented as a pair of curly brackets surrounding zero or more name/value pairs (or members). A name is a string. A single colon comes after each name, separating the name from the value. A single comma separates a value from a following name. The names within an object SHOULD be unique.object = begin-object [ member *( value-separator member ) ] end-object member = string name-separator value
An object whose names are all unique is interoperable in the sense that all software implementations receiving that object will agree on the name-value mappings. When the names within an object are not unique, the behavior of software that receives such an object is unpredictable. Many implementations report the last name/value pair only. Other implementations report an error or fail to parse the object, and some implementations report all of the name/value pairs, including duplicates.
JSON parsing libraries have been observed to differ as to whether or not they make the ordering of object members visible to calling software. Implementations whose behavior does not depend on member ordering will be interoperable in the sense that they will not be affected by these differences.
粗體有描述唯一性，但尷尬的地方在於他用 SHOULD 而非 MUST，所以 library 理論上都要能接受。但後面提到如果不唯一時，行為無法預測 (會到
rm -rf / 嗎？XDDD 最像的應該還是 crash？)，所以還是不要亂來啦...
不過如果真的會 crash 的話，應該也會因為 DoS issue 而被發 CVE，所以實務上應該是不會 crash 啦...