Why Coap Server resend message twice?

Hi Dear
I am working on CoAP in a Node-Red application. In my scenario, the client sends a single request to the CoAP Server every one second . Why do I always receive two responses from the CoAP Server (for each request?) how to make it one response?
note:
i am using function node in the server with this code:
msg.res.end("50")
return msg;