Creating manual threads, also tried using parallel.foreach and async await - but getting duplicate [duplicate]

Answer 1

It's an "Access to modified closure" problem. Look into that for more details. There's a lot of examples of what it is.

Simple fix is to store your n variable in a temp variable everywhere you use it in your delegate.

foreach (int n in arrMessageid)
    {
        int tempN = n;
        Thread thrdSendEmail = new Thread(() =>
        {
                try
                {
                    amazonMessageID = SendSimpleEmail_Part2(messageAmazonRequestBatch.ElementAt(tempN ).req);
                    messageAmazonRequestBatch.ElementAt(tempN ).msg.AmazonMessageID = amazonMessageID;
                    logManager_MessageLogwithAmazonmsgID.LogMessage(",\t" + tempN  , true);
                    //logManager_MessageLogwithAmazonmsgID.LogMessage(",\t" + tempN  + ",\t" + messageAmazonRequestBatch.ElementAt(tempN ).msg.QueueMessageId + ",\t" + amazonMessageID, true);
                }
                catch (Exception ex) { logManager_RunSummary.LogMessage(ex.Message, true); }                                
        });
        thrdSendEmail.Name = n.ToString();
        lstThread.Add(thrdSendEmail);
        thrdSendEmail.Start();
        //logManager_MessageLogwithAmazonmsgID.LogMessage(",\t" + n, true);
    }