Demonstrate how to create a form submit processing script that can handle missing or duplicate trigger firings.

FormSubmitFallback.gs

// Change this values based on your spreadsheet.
var SHEET_NAME = 'Form Responses 1';
var STATUS_COLUMN_NUMBER = 4;
var PROCESSED_STATUS = 'Processed';

var LAST_ROW_KEY = 'lastRow';
var LOCK_TIMEOUT_MS = 60000; // 1 minute
var MAX_RUNTIME_MS = 240000; // 4 minutes

/**
 * Handles form submits as they come in.
 * Configure this function to run on form submit.
 */
function onFormSubmit(event) {
  var range = event.range;
  var sheet = range.getSheet();
  var rowNumber = range.getRow();
  processRow(sheet, rowNumber);
}

/**
 * Checks to make sure all recent form submits were processed.
 * Configure this function to run on a time-based trigger.
 */
function fallback() {
  var start = new Date();
  var properties = PropertiesService.getDocumentProperties();
  var lastRowChecked = Number(properties.getProperty(LAST_ROW_KEY) || 1);
  var sheet = SpreadsheetApp.getActive().getSheetByName(SHEET_NAME);
  var numRows = sheet.getLastRow();
  var rowNumber = lastRowChecked + 1;
  while (rowNumber <= numRows && !isTimeUp(start)) {
    Logger.log(rowNumber);
    processRow(sheet, rowNumber);
    rowNumber++;
  }
  properties.setProperty(LAST_ROW_KEY, rowNumber - 1);
}

/**
 * Processes a single row / form submission in the spreadsheet,
 * ensuring it hasn't already been processed.
 */
function processRow(sheet, rowNumber) {
  var lock = LockService.getDocumentLock();
  lock.waitLock(LOCK_TIMEOUT_MS);
  
  var statusRange = sheet.getRange(rowNumber, STATUS_COLUMN_NUMBER);
  var status = statusRange.getValue();
  if (!status || status.length == 0) {
    
    // Add your code here to actually process the row.
    
    statusRange.setValue(PROCESSED_STATUS);
  }
  
  lock.releaseLock();
}

/**
 * Helper function for determining if the script is close
 * to timing out.
 */
function isTimeUp(start) {
  var now = new Date();
  return now.getTime() - start.getTime() > MAX_RUNTIME_MS;
}