The feature is accessible on the My Bookmarks page (only in desktop browser, not available in mobile browser).
Make sure "All" is selected and no tag is selected from the Available tags list. This is the default state when you click on the "My Bookmarks" menu and the page loads. The "Find Duplicates" menu will be accessible only when the these conditions are met.
Click on the "Find Duplicates" menu to bring up the "Find Duplicates" dialog.
A bookmark is considered to be duplicated if it has the same URL as another bookmark.
Enter a tag (default tag: "duplicate") that you want to add to one of each duplicate pairs.
For example if 5 bookmarks have the same URL then the tag will be added to 4 of them.
To start the search click on the "Find Duplicates" button.
When the search is complete, click on OK to close the message box then click on the "Duplicate" tag in the Available tags list to see the duplicated bookmarks.
Now select all the bookmarks (click on "Select All") and delete them (click on "Delete").
Before deleting the bookmarks always make sure that the "Duplicate" tag has been selected.