Fail early when json fields appear more than once about flatbuffers

[dyu]

Btw, the assertion failure will trigger by editing samples/monsterdata.json to this:

{
  pos: {
    x: 1,
    y: 2,
    z: 3
  },
  hp: 80,
  hp: 80,
  name: "MyMonster"
}

The error on this one would be "struct field appearing out of order: y" (struct failing first/early)

{
  pos: {
    x: 1,
    y: 2,
    y: 2,
    z: 3
  },
  hp: 80,
  hp: 80,
  name: "MyMonster"
}

That's indeed a bug, will create a proper fix.

just fixed.