Add/Update Google Sheet Row Data Comparing Values from HTML Form Using Google Apps Script - Trying to Find Match on Sheet & Update Selected Row Data

I have an HTML form that allows users to populate the form from a Google Sheet, and successfully add new rows to the Sheet. I created the Apps Script according to this tutorial.

I am trying to expand the function of my Apps Script to search the rows of entries for duplicates based on data from two columns and if there is a match, overwrite the existing row.

I know I need a for loop that iterates through each row in the Google Sheet to compare to the form data fields in question, but I don't know how to access each.

This is a shortened example of the the form (the actual is much longer):

<form id="form">
  <form id="form">
  <input name="firstName" placeholder="firstName" />
  <input name="lastName" placeholder="lastName" />
  <input name="someOtherField" placeholder="someOtherField" />
  <input name="someFourthField" placeholder="someOtherField" />
  <div class="btn-success btn" onclick="SaveData()">Save Data</div>
</form>
</form>
<script>
function SaveData() {
  var formData = new FormData(document.getElementById("form"));
  fetch('https://script.google.com/macros/s/AKfycbwQFSXfeOKBHzf41MF6Nh5XIOjaPvr159-blUxsg5smD3BDH8qB4RUZRRo8q9nCJLb18w/exec', 
        {
      method: 'post',
      body: formData,
  })
}
</script>

My Apps Script works perfectly when adding new rows, but either my for loop is not written correctly and thus not finding matches or otherwise setting the nextRow index isn't working:

var sheetName = 'Entries'
var scriptProp = PropertiesService.getScriptProperties()

function intialSetup () {
  var activeSpreadsheet = SpreadsheetApp.getActiveSpreadsheet()
  scriptProp.setProperty('key', activeSpreadsheet.getId())
}

function doPost (e) {
  var lock = LockService.getScriptLock()
  lock.tryLock(10000)

  try {
    var doc = SpreadsheetApp.openById(scriptProp.getProperty('key'))
    var sheet = doc.getSheetByName(sheetName)

    var headers = sheet.getRange(1, 1, 1, sheet.getLastColumn()).getValues()[0]
    var nextRow = sheet.getLastRow() + 1

    var newRow = headers.map(function(header) {
      return header === 'timestamp' ? new Date() : e.parameter[header]
    })

    var range = sheet.getDataRange();
    var rangeData = range.getValues();

    // Here is where the script is failing to find a match in the spreadsheet
    for(i = 1; i > rangeData.length; i++) {
      if(rangeData[i][0] == e.firstName && rangeData[i][1] == e.lastName) 
      {
        nextRow = i;
      }
    }

    sheet.getRange(nextRow, 1, 1, newRow.length).setValues([newRow])

    return ContentService
      .createTextOutput(JSON.stringify({ 'result': 'success', 'row': nextRow }))
      .setMimeType(ContentService.MimeType.JSON)
  }

  catch (e) {
    return ContentService
      .createTextOutput(JSON.stringify({ 'result': 'error', 'error': e }))
      .setMimeType(ContentService.MimeType.JSON)
  }

  finally {
    lock.releaseLock()
  }
}

With the above script, every form entry creates a new row, the match is never true. So I think there is something wrong either with my logic in matchfinding or the syntax I'm using to find the form values and spreadsheet data. So what am I missing?

Thanks!

EDIT: To help elaborate this example, I have attached a screenshot of a sample spreadsheet. As is, if a user submits a form with firstName "Bob" and lastName "Belcher," I would like to have the remaining form data overwrite the existing row with those names. But if the firstName and lastName fields have no match in the spreadsheet, add a new row.

The above script always adds new rows even if there is an existing row. I tried using the logic from this question to achieve my desired result but this caused the script to fail entirely (no updated rows and no added rows). I also tried using the logic steps outlined in this video which uses "indexOf" form data instead of a for loop with if statement to find matches. This solution also did not work

Spreadsheet Screenshot

EDIT EDIT: I have made the whole script available and recorded a short video of the current script behavior versus the desired behavior. Hopefully this clarifies my question. Thanks!