Google Form on Submit, Send Email Lock Doesn't

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?

You can check all the rows in the sheet instead of just the last row and if the row doesn't have the flag set to "Email Sent", send the email.

Not the lock is the problem here, but the fact that you check just the last row.