I ran into this issue too. Like you said, it's not preventing your app from working properly, it's just that I'd like to avoid warnings wherever possible (especially the ones I can not explain!).
In my case I'm running 2 Spring Boot applications simultaneously, which is exactly the reason for this warning. The LiveReload server is then also started twice, while the port it wants to use (35729) can obviously be bound to only once.
So I started looking for a way to change the LiveReload server port for one of my two applications. The best I could find is:
http://livereload.com/tips/change-port-number-livereload-listens-on/
The option where I would need to set an environment variable LRPortOverride
containing the new port number seemed feasible to me. However, no matter what I tried, my environment variable was simply ignored. So no cigar there :-(
Thus, my search continued and I finally found this:
https://docs.spring.io/spring-boot/docs/current/reference/html/using-boot-devtools.html#using-boot-devtools-livereload
This confirms my conclusion and also provides a solution for my problem. I simply added
spring.devtools.livereload.enabled=false
to the Spring application.properties of one of my applications, and the issue was fixed.
Hope this will help you too.