This seems to be common problem for Squarespace users after adding a custom domain name. If Google is indexing the same content, this will result in duplicate content issues, which can impact your SERP.
To prevent this from occurring, rename your user account at Squarespace to something else, since it's no longer needed with a custom domain, as covered here: Removing Squarespace Subdomain from Search Engine Index
To solve the duplicate content issues in the meantime, add a canonical URL to the pages that are being indexed on both domains to let Google know URL's containing your custom domain are the preferred versions.
Regarding redirecting visitors, by changing the Squarespace account as covered above, visitors will no longer see the duplicate indexed content, so there is no need to redirect them. If that is not desirable for some reason, you can create 301 redirects in your Squarespace account for each duplicate URL that is being indexed. See this for how to do that: HOW DO I CREATE URL SHORTCUTS OR MAPPINGS?