For the home page of a domain name, it is not possible to cause duplicate content due to the presence or absence of a trailing slash. That is because the trailing slash is implicitly added by any browser or crawler when a request for the home page is made. It is not possible to make a request for the domain home page without a trailing slash. See: Is trailing slash automagically added on click of home page URL in browser?
Search engines (including Google) know that the domain home page slash is required and would never index the two versions separately.
For folders, it is a different story.
/folder/ can be two different URLs with different content. Serving the same content at both URLs can result in duplicate content. Similarly, the home page URL can have problems when serving the same content as
That being said, Google is much better about handling those common server configuration duplicate content cases than it used to be. Google almost never indexes two URLs with the same content. It picks one and indexes just one. It appears that Google may even combine link juice from external links, so there is no link juice is lost.
In any case, duplicating content on a handful of URLs within your own site is never going to cause penalties. At the worst, Google will pick a URL to put in the results that is not the one that you prefer.
I don't know of any way that spammers can take advantage of duplicate content on similar URLs within your own site.
For more information see: What is duplicate content and how can I avoid being penalized for it on my site?