First thing I tried was the Haskell Aeson library which keeps the first value of the key: λ> decode f :: Maybe Value Just (Object (fromList [("this_art

First thing I tried was the Haskell Aeson library which keeps the first value of the key: λ> decode f :: Maybe Value Just (Object (fromList [("this_article",String "// would be better")])) There's a more flexible jsonWith function that lets you control how duplicates (and, for that matter, field order) is handled, but it's not the default. The handful of other tools I spot-checked all keep the last pair, so it seems like almost but not quite standard behavior. I was curious about other programs and, searching around, found a discussion[1] about standardizing the behavior to "either reject objects with duplicate keys or keep the last value"; however, from a quick scan, that wasn't added to any of the standards documents I found. Apart from the standard I linked earlier there are a couple of JSON RFCs[2][3], but they basically say "different systems do different things and if you depend on the behavior, your JSON won't be universally interoperable" > 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. If anything, the fact that it is almost but not quite standard behavior probably makes it worse—you can rely on it for quite a while until it unexpectedly breaks and the figuring out why it broke will be a real headache. [1]: https://esdiscuss.org/topic/json-duplicate-keys https://esdiscuss.org/topic/json-duplicate-keys [2]: https://datatracker.ietf.org/doc/html/rfc7159 https://datatracker.ietf.org/doc/html/rfc7159 [3]: https://datatracker.ietf.org/doc/html/rfc8259 https://datatracker.ietf.org/doc/html/rfc8259