As the comment in your code says:
// port number 3389 reserved as RDP listener port.
this is standard RDP port, and if you have RDP enabled on your computer the RDP service is using this port.
The solution is to disable/stop RDP service or change the default listening port.
If you only want to monitor for new connections you could use GetActiveTcpConnections
var RdpConnections = IPGlobalProperties.GetIPGlobalProperties().GetActiveTcpConnections().Where(c => c.LocalEndPoint.Port == 3389);
Call this periodically to get new connections.
Loop through RdpConnections and look at their RemoteEndPoint for address.