The domain name is already added to a hosting account on our platform. Please remove your domain nam

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Seem like you are adding a subdomain.
This may be more applicable to you

Hosting only a custom subdomain with InfinityFree

Hello! This guide will explain how you can host a subdomain of a domain name you own with InfinityFree without hosting the main domain too. Here is the example I will use in the guide. Note: I will be using terms that you may not recognize. Please read the first section of this article on domain names if you get confused. You own the domain example.com and want to add the subdomain blog.example.com to InfinityFree. However, you do not want example.com to be hosted with InfinityFree. Here is…