App script to prevent duplicate form submission

Question

I have a gform where the 2nd question is mobile number. I want to make sure the same mobile number is not entered more than once. Responder should get an error when submitting that the number is already registered. How to use app script to do this. I have tried following code,which is executed successfully but didn’t work.

function onFormSubmit(e) {
  var form = FormApp.getActiveForm();
  var itemResponses = e.response.getItemResponses();
  var newMobile = itemResponses[1].getResponse();
  
  var previousResponses = form.getResponses();
  var isDuplicate = false;
  
  // Check if the new mobile number is a duplicate
  for (var i = 0; i < previousResponses.length; i++) {
    var previousItemResponses = previousResponses[i].getItemResponses();
    var previousMobile = previousItemResponses[1].getResponse();
    if (newMobile === previousMobile) {
      isDuplicate = true;
      break;
    }
  }
  
  // If duplicate, show an error message and delete the response
  if (isDuplicate) {
    form.setCustomClosedFormMessage("This mobile number has already been submitted. Please enter a different mobile number.");
    form.deleteResponse(e.response);
  }
}