I am having this issue "“Another site is already using this domain” error trying to set custom domain “www.jonin.gq” on site “inspiring-swartz-ccf57c”. I can verify the domain if needed.
My Domain: www.jonin.gq
My Site: https://dazzling-beaver-8ecfe6.netlify.app
Hey @Mayu, welcome to the forum 
Are you still having this issue? I see that the domain is now attached to a site deploying a GitHub repo that shares your username…
If this happens to be an incredible coincidence and this is still not resolved, please let us know!
I was able to figure out which account it was connected to and asked them to remove it. The issue is now resolved. Thanks for responding