'Same HTTPS binding' error using IIS

If you only have one IP address, you need to set all sites that are using HTTPS to require Server Name Identification so that IIS knows what cert to use for clients connecting to which site. Either each site needs to use SNI, or you must use different ports, or different IP addresses.

By default most webservers assume that if you're coming to them with HTTPS, you want the main site, and they'll happily serve that to you and rely on you to make sure the cert matches the domain. When you're hosting multiple sites over TLS/SSL, there's no way for the server to send you the proper certificate if you don't supply any information about what you're expecting. SNI is a TLS extension which allows a client to say "Hi, I'm looking for https://unitconversions.net" and a server to go "Thanks, here's the cert for that site, and have a good connection."

The warning you're seeing is IIS making sure you don't break your other sites that are listening on all IP addresses for inbound HTTPS connections.

More information about the IIS implementation of SNI can be found HERE.