When it comes to the home page of a domain, it is the one place in which the trailing slash doesn't matter too much.
are the same url. There is no duplicate content issue.
When a web browser requests them, it has to send a request that looks like:
GET / HTTP/1.1
This is different than
in which the URLs could be configured to serve different content.
Having the slash on the end of the domain link makes it more like what the request that gets sent looks like and makes it seem more like the subdomain case. However it adds complexity (and one character to the url). I think it is a style issue more than a canonical/duplicate content issue. Its a similar issue as to whether or not the domain name is capitalized http://example.com vs http://example.com. It doesn't matter in terms of content.