It prevents the same event ID from being executed more than once

preventsSameEventID.js

var alreadyRunEventIDs = [];

function isAlreadyRunning(eventID) {
  return alreadyRunEventIDs.indexOf(eventID) >= 0;
}

function markAsRunning(eventID) {
  alreadyRunEventIDs.push(eventID);
}

exports.notifyNewPost = functions.firestore.document('path').onCreate(event => {
  const eventID = event.eventId;
  if (util.isAlreadyRunning(eventID)) {
    return console.log('Ignore it because it is already running (eventId):', eventID);
  }
  markAsRunning(eventID);
  ...
}