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Domain is owned by a different organization.
I added my domain to a project, then removed it to migrate it to another project. However, I'm now unable to add it again it says the domain already exists.
TL;DR
Developer added a domain to a project, then removed it to migrate to another project. Unable to re-add the domain due to error message it already exists. Solution: Check other projects to ensure domain removal was successful before re-adding.