First you have to find the duplicates. It would be nice if Google told you in Search Console which URL it thinks is duplicate, but they don't divulge this information as far as I've been able to tell.
To find the duplicate you instead usually have to use Google search. Take a phrase from the page that should be unique and search for it on Google with quotes around it. That should tell you which page Google is indexing instead.
Once you know where the duplicate is you can make a decision about what to do with it. You will have several options:
- Do nothing and let Google choose which page it thinks is duplicate on its own. Google usually only indexes one copy of a page when it finds duplicates. Having duplicate pages and letting Google deal with them automatically doesn't usually hurt your site. See What is duplicate content and how can I avoid being penalized for it on my site?
- If you have control over both pages you can use canonical tags or redirects to tell Google which one you would prefer it indexes.
- If the duplicate content is on some other site and the content belongs to you, you may want to try to get the other site shut down or removed from Google using DMCA.
- You could also differentiate the pages more so they are no longer duplicate.
Edit: As suggested in the comments, you can also find the duplicate by using the "View as Search Result" link in Search Console that is available when you click on the URL in the report:
