Google Apps Script failing onFormSubmit with concurrent responses - google-apps-script

Question

I've setup a Google Form and linked Sheet to record form responses and then create and email the respondent a letter from the submitted data. This has been working fine and has started getting quite popular among colleagues, but now a problem has arisen with onFormSubmit when two users submit the form at the same time.
Essentially, the script is setup to run on the last row of the sheet onFormSubmit but if two responses are submitted concurrently it only runs the script on the last entry, potentially twice.
I'm thinking it might be possible to get around this by setting up a column to be marked if the script completed and then setting up a time-drive trigger to run on any rows that haven't been marked but this feels a bit clunky as respondents usually require the letter immediately.
Is there another way to approach the problem so that onFormSubmit makes sure the script actually runs on the row created by the original form submission and not simply the last row? Any help would be much appreciated, here's an example of my code:
function createLetterFromForm(){
// Get data from sheet
var ss = SpreadsheetApp.getActiveSpreadsheet();
var sheet = ss.getActiveSheet();
// Define range and data in each column
var data = sheet.getRange(sheet.getLastRow(), 1, 1,
sheet.getLastColumn()).getValues(); // Range (last entry submitted)
for (var i in data){
var row = data[i];
// Pick the right template
if (row[9]=="A"){
var templateid = "xxxxxxxxxxxx";} // Template 1
if (row[9]=="B"){
var templateid = "xxxxxxxxxxxy";} // Template 2
// Make copy and set active
var folder = DriveApp.getFolderById("zzzzzzzzzzzzzz") // Folder for generated letters
var docid = DriveApp.getFileById(templateid).makeCopy(row[7]+" - Letter",folder).getId();
var doc = DocumentApp.openById(docid);
var docBody = doc.getActiveSection();
// Copy data to template
// address
docBody.replaceText("%FNAME%", row[2]);
docBody.replaceText("%SNAME%", row[3]);
docBody.replaceText("%ADDL1%", row[4]);
docBody.replaceText("%ADDL2%", row[5]);
docBody.replaceText("%ADDL3%", row[6]);
docBody.replaceText("%PCODE%", row[7]);
// other data
docBody.replaceText("%DATA1%, row[8]);
// etc.
// Share and Save doc
doc.addEditor(row[1]);
doc.saveAndClose();
// Email PDF to Respondee
var sendFile = DriveApp.getFilesByName(row[7]+' - Letter');
var recipient = row[1]
MailApp.sendEmail({
to:recipient,
subject: "Your Letter",
body:"Hello, \n\nHere's a PDF copy of the letter you created.",
attachments: [sendFile.next()]
});
}
}