Clicking on the “Duplicate Items” collection in your library or right-clicking the library in the left pane and selecting “Show Duplicates” will show the items Zotero thinks are duplicates in the center pane.

In Zotero, it is best to resolve duplicate items by merging them, rather than deleting one of the duplicates.

To merge items in the “Duplicate Items” collection, select an item in the center pane. Zotero will automatically co-select the other items that it thinks are duplicates. Click the “Merge <number> Items” button in the right pane to merge the items. If the item fields don't match completely, you can select one item to be the “master” from the list at the top of the right pane, then select alternative versions of mismatched fields using the icons to the right of each field.

Full instructions: https://www.zotero.org/support/duplicate_detection