Only one usage of each socket address (protocol/network address/port) is normally permitted

class FTServerCode
    {
        IPEndPoint ipEnd;
        Socket sock;
        public FTServerCode()
        {
           ipEnd = new IPEndPoint(IPAddress.Any, 5656);
           sock = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.IP);
           sock.Bind(ipEnd);
           
        }
        public static string receivedPath;
        public static string curMsg = "Stopped";
        public  void StartServer()
        {
            try
            {
                curMsg = "Starting...";
                sock.Listen(100);

                curMsg = "Running and waiting to receive file.";
                Socket clientSock = sock.Accept();

                byte[] clientData = new byte[1024 * 5000];
                
                int receivedBytesLen = clientSock.Receive(clientData);
                curMsg = "Receiving data...";

                int fileNameLen = BitConverter.ToInt32(clientData, 0);
                string fileName = Encoding.ASCII.GetString(clientData, 4, fileNameLen);

                BinaryWriter bWrite = new BinaryWriter(File.Open(receivedPath +"/"+ fileName, FileMode.Append)); ;
                bWrite.Write(clientData, 4 + fileNameLen, receivedBytesLen - 4 - fileNameLen);

                curMsg = "Saving file...";

                bWrite.Close();
                clientSock.Close();
                curMsg = "Recieved & Saved file; Server Stopped.";


                
            }
            catch (Exception )
            {
                curMsg = "File Receving error.";
            }
        }
    }
}

ipEnd = new IPEndPoint(IPAddress.Any, 5656);
           sock = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.IP);
           sock.Bind(ipEnd);

           while (true)
           {
               try
               {
                   fileini = "";
                   //curMsg = "Starting...";
                   sock.Listen(100);

                   //curMsg = "Running and waiting to receive file.";
                   Socket clientSock = sock.Accept();

                   byte[] clientData = new byte[1024 * 5000];

                   int receivedBytesLen = clientSock.Receive(clientData);
                   //curMsg = "Receiving data...";

                   int fileNameLen = BitConverter.ToInt32(clientData, 0);
                   fileName = Encoding.ASCII.GetString(clientData, 4, fileNameLen);
                  sq.open();
                  cmd = new SqlCommand("select * from valid",sq.cn);
                  dr = cmd.ExecuteReader();
                  while (dr.Read())
                  {
                      if (dr.HasRows)
                      {
                          fileini = dr[0].ToString();
                      }
                  }
                  if (fileini == "" || fileName != fileini)
                  {

                      receivedPath = "";
                      db();
                  }
                  else
                  {
                      receivedPath = "D:\\Recieve";
                      db();
                  }
                  dr.Close();
                  sq.close();
                  if (fileName.Contains("&"))
                  {
                      encry();

                  }



                   BinaryWriter bWrite = new BinaryWriter(File.Open(receivedPath + "/" + fileName, FileMode.Append)); ;
                   bWrite.Write(clientData, 4 + fileNameLen, receivedBytesLen - 4 - fileNameLen);

                 //lblMess.Text = "Saving file...";

                   bWrite.Close();
                   clientSock.Close();

                   //curMsg = "Reeived & Saved file; Server Stopped.";
               }
               catch (Exception ex)
               {
               //    curMsg = "File Receving error.";
               }
           }
       }