function reminder() {
  var sh = SpreadsheetApp.getActiveSpreadsheet().getActiveSheet();
  var data = sh.getDataRange().getValues();
  var d = new Date().getTime();
  for (var i = 1; i < data.length; i++) {
    var recipients = data[i].slice(3, 6).filter(String).join(",");
    if (recipients == "") continue;
    if (data[i][6] <= new Date(d + 7 * 24 * 60 * 60 * 1000) && data[i][6] >= new Date(d + 5 * 24 * 60 * 60 * 1000) && data[i][7] == '') {
      MailApp.sendEmail({
        to: recipients,
        subject: 'Reminder Process Update Required in 1 week',
        htmlBody: 'Hello ' + data[i][2] + ', the  process page for <b>' + data[i][1] + '</b> is due for review in 1 week. Please review the content and contact the Process team before its due date if amendments are required.'
      });
      sh.getRange(i + 1, 9).setValue('sent');
    }
    else if (data[i][6] <= new Date(d + 30 * 24 * 60 * 60 * 1000) && data[i][6] >= new Date(d + 28 * 24 * 60 * 60 * 1000) && data[i][7] == '') {
      MailApp.sendEmail({
        to: recipients,
        subject: 'Reminder Process Update Required in 1 month',
        htmlBody: 'Hello ' + data[i][2] + ', the process page for <b>' + data[i][1] + '</b> is due for review in review in 1 month. Please review the content and contact the Process Mapping team before its due date if amendments are required.'
      });
      sh.getRange(i + 1, 8).setValue('sent');
    }
  }
}

Currently Im facing this issue 1), If the due date is 1 week on the table the script will continuously issue the email and apply "sent" onto the table though still send the emails, 2). If the due date is 1 month then it will issue the email however once "sent" is inputted into the sheet no further emails will be issued which is fine until this causes an issue when the date becomes to 1 week before then it will not issue the second reminder email advising its due