JSON fields have the same name

Question

In practice, keys have to be unique within a JSON object (e.g. Does JSON syntax allow duplicate keys in an object?). However, suppose I have a file with the following contents:

{
    "a" : "1",
    "b" : "2",
    "a" : "3"
}

Is there a simple way of converting the repeated keys to an array? So that the file becomes:

{
    "a" : [ {"key": "1"}, {"key": "3"}],
    "b" : "2"
}

Or something similar, but which combines the repeated keys into an array (or finds and alternative way to extract the repeated key values).

Here's a solution in Java: Convert JSON object with duplicate keys to JSON array

Is there any way to do it with awk/bash/python?

Answer 1

If your input is really a flat JSON object with primitives as values, this should work:

jq -s --stream 'group_by(.[0]) | map({"key": .[0][0][0], "value": map(.[1])}) | from_entries'

{
  "a": [
    "1",
    "3"
  ],
  "b": [
    "2"
  ]
}

For more complex outputs, that would require actually understanding how --stream is supposed to be used, which is beyond me.