Looks like I was beaten to it but it does sound as though its just another domain pointing to your IP rather than a complete clone.
Its happened many times - you need to ensure your running https
https://meta.discourse.org/t/someone-elses-domain-is-pointing-to-my-forum-are-there-measures-i-can-take-to-block/70820?u=mikechristopher
https://meta.discourse.org/t/other-domain-names-are-pointed-to-my-ip-address/92342?u=mikechristopher