fcf-check-eventID.ts

export async function wasTriggered(eventId: string) {
  return firestore.runTransaction(async t => {
    const ref = eventIdRef
    const doc = await t.get(ref)
    if (doc.exists) {
      return true
    } else {
      t.set(ref, {})
      return false
    }
  })
}

export const listener = functions.firestore.document(`path`).onCreate(async (snapshot, context) => {
  if (await utils.wasTriggered(context.eventId)) {
    console.log('Ignore: it has already been triggered')
    return 0
  }
  ...
}