firestore cloud functions onCreate/onDelete sometimes immediately triggered twice

See the Cloud Firestore Triggers Limitations and Guarantees:

Delivery of function invocations is not currently guaranteed. As the Cloud Firestore and Cloud Functions integration improves, we plan to guarantee "at least once" delivery. However, this may not always be the case during beta. This may also result in multiple invocations for a single event, so for the highest quality functions ensure that the functions are written to be idempotent.

There is a Firecast video with tips for implementing idempotence.

Also two Google Blog posts: the first, the second.

Based on @saranpol's answer we use the below for now. We have yet to check if we actually get any duplicate event ids though.

const alreadyTriggered = eventId => {
  // Firestore doesn't support forward slash in ids and the eventId often has it
  const validEventId = eventId.replace('/', '')

  const firestore = firebase.firestore()
  return firestore.runTransaction(async transaction => {
    const ref = firestore.doc(`eventIds/${validEventId}`)
    const doc = await transaction.get(ref)
    if (doc.exists) {
      console.error(`Already triggered function for event: ${validEventId}`)
      return true
    } else {
      transaction.set(ref, {})
      return false
    }
  })
}

// Usage
if (await alreadyTriggered(context.eventId)) {
  return
}