If we provide a link to a Catalog entry, and people who are already enrolled in the Canvas course use that link, they are taken into Catalog and see a meesage "You are enrolled". There is no link to the Course site though.
This is not a great user experience. We would actually be better providing the Self Enrol link for the course site, but that misses any additional information that is provided in the Catalog entry.
This screenshot is an example of a catalog entry, with the "You are enrolled" message displayed.