Duplicate required() generates invalid schema

[boyander]

🐛 Bug Report

Calling required() twice on same prop duplicates key in required schema array required:[<prop>,<prop>] generating an invalid schema.

To Reproduce

Steps to reproduce the behavior:

const S = require('fluent-json-schema')

const schema = S.object()
  .prop('email', S.string()).required().required()
  
console.log(JSON.stringify(schema.valueOf(), undefined, 2))

causes invalid schema with duplicate prop in required array

{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "type": "object",
  "properties": {
    "email": {
      "type": "string"
    }
  },
  "required": [
    "email",
    "email"
  ]
}

https://replit.com/@boyander/duplicatedrequiredprop

Expected behavior

Either throw an error if using require() twice or generate the required:[] array using a Set

Your Environment

• node version: 12
• os: Mac

Also, congrats for this awesome package. We use it a lot at @core-school

[mcollina]

We are glad you like it!

Would you like to send a Pull Request to address this issue? Remember to add unit tests.

[boyander]

Sure! Will work on it this weekend ☀️