const sampleSize = 2
const randomIndex = (array) => Math.floor(Math.random() * array.length)

this.moduleCollection$.doc(module.id).collection('questions').valueChanges()
    .map(r => r[randomIndex(r)])
    .repeat()
    .scan((a, c) => a.indexOf(c) === -1 ? a.concat(c) : a, [])
    .skipWhile(array => array.length < sampleSize)
    .take(1)
    .subscribe(res=>{
      console.log(res);
  })