/page and /page/ Are Different URLs
Technically, `example.com/about` and `example.com/about/` are two different URLs. They can serve different content.
In practice, most servers treat them the same. But "most" is not "all."
What to Do
Pick one. Redirect the other. Either trailing slash or no trailing slash. It does not matter which you choose. What matters is consistency.
If both versions return 200, you have duplicate content. Every page on your site has two URLs.
Set up a server-level redirect so one version always redirects to the other. Then make sure all your internal links, canonical tags, and sitemap entries use the chosen version. This is also covered in our URL structure best practices post. Google's SEO documentation treats trailing and non-trailing slash URLs as separate pages.
That Is It
This is not complicated. But it is frequently overlooked. And it is on our 113-task checklist for exactly that reason. Free. 30 seconds.