Google Form on Submit, Send Email Lock Doesn't Work

问题

I have created a google form with an email field. Responses are stored in a spreadsheet. On submit I take the last row from the sheet (latest response) and send an email to that user.

I have tried to implement a lock system, however it does not work the way I would like it too. If a there are multiple submissions in quick succession (i.e 3 within a minute) only the last user will get an email sent to them, they will in fact get 3 duplicate emails the number submitted in quick succession.

function EmailFormConfirmation() {
// Get a public lock on this script, because we're about to modify a shared resource.
var lock = LockService.getScriptLock()
// Wait for up to 30 seconds for other processes to finish.
lock.waitLock(30000);
 try {
var sheetname = "reg-responses"
var columnnumber = 4

var ss = SpreadsheetApp.getActiveSpreadsheet(); 
var sheet = ss.getSheetByName(sheetname);

// Determines row number of most recent form submission and sets it as "lastrow"
if (sheet.getRange(sheet.getMaxRows(),1).getValue() != "") {
var lastrow = sheet.getMaxRows()
    } 
else {
  var count = 0
  for (var i = 0; i < sheet.getMaxRows(); i++) {
    if (sheet.getRange(sheet.getMaxRows()-i,1).getValue() != "") {
      var lastrow = sheet.getMaxRows()-i
      break;
    }  
  }
 }
  var email = sheet.getRange(lastrow,columnnumber).getValue();                            
  var name = sheet.getRange(lastrow,2).getValue();

  var message = "<HTML><BODY>"
    +"<h3>Your registration has been submitted</h3>"
    +"<p><b>Name</b>: "+name+"</p>"        
    +"</HTML></BODY>";      

  //validation is handled on the form as regex
 MailApp.sendEmail(email, "Form Submitted", "", {htmlBody: message}); 

 //flags it as email sent
 sheet.getRange(lastrow,8,1,1).setValue("Email Sent");

         //Here is the catch if the lock doesn't work
  if(!lock.hasLock()){

     Logger.log('Could not obtain lock after 30 seconds.');
  }
//Secondary catch for the entire function. 
} catch (e) {
Logger.log(e.toString());
}

 // Release the lock so that other processes can continue.
 lock.releaseLock();
}

What issue must I fix on my lock to resolve this?