Help - Google indexed URLs both w/ or w/o trailing slash

For some reason Google indexed one of my blog posts with BOTH trailing slash (/) at the end of the URL and without.

URLs without trailing slash at the end of the URL automatically redirects to the one with / in the end. The one with the / in the end is the correct one as per my WordPress settings.

But I can imagine this creates a duplicate content issue. Why did Google index my blog post without the trailing slash as well? How can I fix this?