A custom domain can be linked to only one Glitch project.  

This means:

• If you see the message, this custom domain is already registered, the custom domain that you are trying to add to your project has already been linked to a different Glitch project. 
• When you see this error, you will need to remove the custom domain from the project it is currently connected to before you can add it to another project.  Note: as our custom domain feature is temporarily unavailable, if you remove any of your custom domains, you will not be able to add it back
• Archiving a project does not remove custom domains that are connected to it. Before you can remove a custom domain from a project, you must un-archive it.  Don’t worry, you can re-archive it when you are done.
• If you need help figuring out which project your custom domain is connected to, you can send us a support request.  We can look it up for you!