standards - Does JSON syntax allow duplicate keys in an object?

Question

Is this valid json?

{
    "a" : "x",
    "a" : "y"
}

http://jsonlint.com/ says yes.

http://www.json.org/ doesn't say anything about it being forbidden.

But obviously it doesn't make much sense, does it? Most implementations probably use a hashtable so it is being overriden anyways.

Answer 1

The short answer: Yes but is not recommended.
The long answer: It depends on what you call valid...

[ECMA-404][1] "The JSON Data Interchange Syntax" doesn't say anything about duplicated names (keys).
However, [RFC 8259][2] "The JavaScript Object Notation (JSON) Data Interchange Format" says:

The names within an object SHOULD be unique.

In this context SHOULD must be understood as specified in BCP 14:

SHOULD This word, or the adjective "RECOMMENDED", mean that there may exist valid reasons in particular circumstances to ignore a particular item, but the full implications must be understood and carefully weighed before choosing a different course.

[RFC 8259][2] explains why unique names (keys) are good: > 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.

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Also, as Serguei pointed out in the comments: ECMA-262 "ECMAScript? Language Specification", reads:

In the case where there are duplicate name Strings within an object, lexically preceding values for the same key shall be overwritten.

In other words, last-value-wins.

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Trying to parse a string with duplicated names with the Java implementation by Douglas Crockford (the creator of JSON) results in an exception:

org.json.JSONException: Duplicate key "status"  at
org.json.JSONObject.putOnce(JSONObject.java:1076)