BlockingCollection Consumer is repeating output

Question

TL;DR I have an application that is reading messages from a USB device in the background, and displaying the messages on the screen. I am using a BlockingCollection, as I need to read messages quickly so the device does not get a BufferOverflow.

I am reading messages like this (my producer):

private void ReadMessages(BlockingCollection<object> logMessages)
{
   uint numMsgs;
   Status status;
   Message[] msgs = new Message[10];
   while(!logMessages.IsAddingCompleted)
   {
      numMsgs = (uint) msgs.Length;
      status = readMessages(channel, msgs, ref numMsgs, 1000);
      if(status == Status.ERR_BUFFER_OVERFLOW)
      {
         logMessages.Add("BUFFER OVERFLOW - MESSAGES LOST!");
         logMessages.Add(CopyMessages(msgs, numMsgs));
      }
      else if(status == Status.STATUS_NOERROR)
      {
         logMessages.Add(CopyMessages(msgs, numMsgs));
      }
      else
      {
         throw new Exception("Error");
      }
}

The readMessages() method will fill the msgs array with the Message objects read, and the numMsgs reference holds the number of messages that were read (up to 10). I use a function called CopyMessages() so I only pass a Message[] that is the right size. i.e. if 5 messages are read, I send a Message[5] instead of Message[10].

I read the messages (my consumer) like this:

private void DisplayMessages(BlockingCollection<object> messages)
{
    string[] msgs;
    try
    {
        foreach (var item in messages.GetConsumingEnumerable(_cancellationTokenSource.Token))
        {
            if (item is string)
            {
                msgs = new string[] { item.ToString() };
            }
            else if (item is PassThruMsg[])
            {
                msgs = FormatMessages((PassThruMsg[])item);
            }
            else
            {
                msgs = new string[0];
            }

            Task.Factory.StartNew(new Action(() => outputTextBox.AppendText(String.Join(Environment.NewLine, msgs) + Environment.NewLine)), _cancellationTokenSource.Token, TaskCreationOptions.None, uiContext);
        }
    }
    catch (OperationCanceledException)
    {
        //TODO:
    }
}

I start the tasks inside a button click, like this:

var results = new BlockingCollection<object>();

var display = Task.Factory.StartNew(() => DisplayMessages(results));
var readMessages = Task.Factory.StartNew(() => ReadMessages(results));

Task[] tasks = new Task[] { display, readMessages };

try
{
    await Task.Factory.ContinueWhenAll(tasks, result => { results.CompleteAdding(); }, _cancellationTokenSource.Token, TaskContinuationOptions.None, uiContext);
}
catch (TaskCanceledException)
{
    //TODO:
}

This works fine, and when running idly it prints the messages from the device without a problem. However, after the device starts doing work under a really heavy load (the consumer is called so quickly it locks the UI temporarily) that I notice the output textbox is repeating values. It is my understanding that GetConsumingEnumerable() also removes items from the blocking collection, but I don't know why else I would see the messages printed multiple times. Each message has a timestamp, and when I readMessages from the device it clears the buffer so I know that I am not reading that message multiple times.

Am I missing something here? Is there a better way to handle this producer/consumer scenario to ensure accurate data? I have looked to see if there are references somewhere that may be overlapping, but I don't see it.