ST_AsGeoJSON should warn about duplicate keys

[tobwen]

description

Although all JSON specifications (RFC 8259, RFC 7159, RFC 4627, and ECMA-404) allow multiple columns with the same name, PostgreSQL's JSONB does not. JSONB just takes the last occurance.

Most validators warn about duplicate columns, maybe PostGIS should warn about this, too? It might result in expected behaviour when storing as JSONB.

example

SELECT
    ST_AsGeoJSON(data.*)
FROM
    (SELECT
        1 AS column,
        2 AS column,
        ST_Transform(geom, 3035) geom
    FROM
        ST_SetSRID(ST_MakePoint(7, 51), 4326) geom
    ) data;

result JSON

{
  "type":"Feature",
  "geometry":{
    "type":"Point",
    "crs":{
      "type":"name",
      "properties":{
        "name":"EPSG:3035"
      }
    },
    "coordinates":[
      4110471.051819585,
      3103060.820290524
    ]
  },
  "properties":{
    "column":1,
    "column":2
  }
}

result JSONB

{
  "type":"Feature",
  "geometry":{
    "crs":{
      "type":"name",
      "properties":{
        "name":"EPSG:3035"
      }
    },
    "type":"Point",
    "coordinates":[
      4110471.051819585,
      3103060.820290524
    ]
  },
  "properties":{
    "column":2
  }
}