Okay, this is very weird, Google indexed my pages and some are duplicates (one w/ the trailing slash, and one without). Is this bad for SEO?
The URL's are like:
If two URLs pull up the same content then you will run into issues with duplicate content. The solution to solve this is canonical URLs. Choose which URL you want to show up in Google search and make that your primary URL.
It will cause the issue with the duplicate content, so it is bad for SEO. You may solve it with canonical urls or 301 redirect.
Regarding canonical urls, you should choose the main page from "/" or without "/". If your main page is, e.g. site.com/posts/indexed-post/ on the page site.com/posts/indexed-post you should add in header construction like John Conde shows. But you should do it only for site.com/posts/indexed-post