throw a warning when there are repeated properties

[Gabriel Aumala]

I was validating a eslint json config like this:

{
  "parserOptions": {
    "ecmaVersion": 6,
    "sourceType": "module"
  },
  "rules": {
    "semi": 0,
    "indent": ["warning", 2],
    "comma-dangle": ["error", "always"],
    "no-unreachable": "error",
    "curly": 1,
    "block-scoped-var": 2,
    "no-else-return": 1,
    "no-use-before-define": 2,
    "no-undef": 2,
    "no-var": 2,
    "no-duplicate-imports": 2,
    "no-const-assign": 2,
    "no-dupe-class-members": 2,
    "no-labels": 2,
    "no-extra-parens": 1,
    "indent": 1,
    "no-mixed-spaces-and-tabs": 1,
    "camelcase": 1,
    "prefer-const": 1,
    "max-len": [2, 100],
    "max-statements": [2, 50],
    "no-unused-vars": ["error", { "vars": "all", "args": "none" }]
  },
  "env": {
    "node": true
  }
}

The indent property is defined twice. It is indeed a valid JSON file, but it would be nice if it threw a warning it.