Ignore same-thread emails that have different labe

I am writing the Date and Subject from specific new emails to a new row of a Google Sheet.

I apply a label to the new mail items with a filter.
the script processes those labeled emails
the label is removed
A new label is applied, so that these emails won't be processed next time.

Problem: When there is a myLabel email, the script processes all emails in the same thread (eg same subject and sender) regardless of their label (even Inbox and Trash).

Question: How to only process new emails i.e. ones with the label myLabel - even when the thread of those messages extends outside the myLabel folder?

My current script:

function fetchmaildata() {
  var ss = SpreadsheetApp.getActiveSpreadsheet();
  var sheet = ss.getSheetByName('mySheetName');

  var label = GmailApp.getUserLabelByName('myLabel');
  var threads = label.getThreads();

  for (var i = 0; i < threads.length; i++)
  {
    var messages = threads[i].getMessages();

    for (var j = 0; j < messages.length; j++)
    {     
      var sub = messages[j].getSubject();
      var dat = messages[j].getDate();

      ss.appendRow([dat, sub])
    }
    threads[i].removeLabel(label);
    threads[i].addLabel(newlabel);
  }
}

I hacked a solution for my purposes by changing my for loop to this:

for (var j = messages.length-1; j > messages.length-2; j--)

This says to process only the latest email in the thread, even when there is more than one email of a thread in the myLabel folder. Oddly, the script still changes the Labels of all the myLabel emails, but only the latest one of a thread gets written to the spreadsheet, so it works for me.

I had to make another change to the code because the above code does not run as a time-triggered scheduled task. I changed the code in this way and it now runs on a time schedule !!

//var ss = SpreadsheetApp.getActiveSpreadsheet();  
var ss = SpreadsheetApp.openById("myGoogleSheetID");

A label can be on a thread due to being on a single message in said thread. Your code simply goes label -> all label threads -> all thread messages, rather than accessing only the messages in a thread with a given label. That's not really your fault - it's a limitation of the Gmail Service. There are two approaches that you can use to remedy this behavior:

The (enable-before-use "advanced service") Gmail REST API

The REST API supports detailed querying of messages, including per-message label status, with Gmail.Users.Messages.list and the labelIds optional argument. For example:

// Get all messages (not threads) with this label:
function getMessageIdsWithLabel_(labelClass) {
  const labelId = labelClass.getId();
  const options = {
    labelIds: [ labelId ],
    // Only retrieve the id metadata from each message.
    fields: "nextPageToken,messages/id"
  };
  const messages = [];

  // Could be multiple pages of results.
  do {
    var search = Gmail.Users.Messages.list("me", options);
    if (search.messages && search.messages.length)
      Array.prototype.push.apply(messages, search.messages); 
    options.pageToken = search.nextPageToken;
  } while (options.pageToken);

  // Return an array of the messages' ids.
  return messages.map(function (m) { return m.id; });
}

Once using the REST API, there are other methods you might utilize, such as batch message label adjustment:

function removeLabelFromMessages_(messageIds, labelClass) {
  const labelId = labelClass.getId();
  const resource = {
    ids: messageIds,
    // addLabelIds: [ ... ],
    removeLabelIds: [ labelId ]
  };
  // https://developers.google.com/gmail/api/v1/reference/users/messages/batchModify
  Gmail.Users.Messages.batchModify(resource, "me");
}

Result:

function foo() {
  const myLabel = /* get the Label somehow */;
  const ids = getMessageIdsWithLabel_(myLabel);
  ids.forEach(function (messageId) {
    var msg = GmailApp.getMessageById(messageId);
    /* do stuff with the message */
  });
  removeLabelFromMessages_(ids, myLabel);
}

Tracked Processing

You could also store each message ID somewhere, and use the stored IDs to check if you've already processed a given message. The message Ids are unique.

This example uses a native JavaScript object for extremely fast lookups (vs. simply storing the ids in an array and needing to use Array#indexOf). To maintain the processed ids between script execution, it uses a sheet on either the active workbook, or a workbook of your choosing:

var MSG_HIST_NAME = "___processedMessages";
function getProcessedMessages(wb) {
  // Read from a sheet on the given spreadsheet.
  if (!wb) wb = SpreadsheetApp.getActive();
  const sheet = wb.getSheetByName(MSG_HIST_NAME)
  if (!sheet) {
    try { wb.insertSheet(MSG_HIST_NAME).hideSheet(); }
    catch (e) { }
    // By definition, no processed messages.
    return {};
  }
  const vals = sheet.getSheetValues(1, 1, sheet.getLastRow(), 1);
  return vals.reduce(function (acc, row) {
    // acc is our "accumulator", and row is an array with a single message id.
    acc[ row[0] ] = true;
    return acc;
  }, {});
}
function setProcessedMessages(msgObject, wb) {
  if (!wb) wb = SpreadsheetApp.getActive();
  if (!msgObject) return;
  var sheet = wb.getSheetByName(MSG_HIST_NAME);
  if (!sheet) {
    sheet = wb.insertSheet(MSG_HIST_NAME);
    if (!sheet)
      throw new Error("Unable to make sheet for storing data");
    try { sheet.hideSheet(); }
    catch (e) { }
  }

  // Convert the object into a serializable 2D array. Assumes we only care
  // about the keys of the object, and not the values.
  const data = Object.keys(msgObject).map(function (msgId) { return [msgId]; });
  if (data.length) {
    sheet.getDataRange().clearContent();
    SpreadsheetApp.flush();
    sheet.getRange(1, 1, data.length, data[0].length).setValues(data);
  }
}

Usage would be something like:

function foo() {
  const myLabel = /* get label somehow */;
  const processed = getProcessedMessages();
  myLabel.getThreads().forEach(function (thread) {
    thread.getMessages().forEach(function (msg) {
      var msgId = msg.getId();
      if (processed[msgId])
        return; // nothing to do for this message.
      processed[msgId] = true;
      // do stuff with this message
    });
    // do more stuff with the thread
  });
  setProcessedMessages(processed);
  // do other stuff
}