It's best to have only one copy of each reference in your library. You can view duplicates Zotero has automatically identified by selecting the Duplicate Items special collection under My Library.
In Duplicate Items, you'll see multiple copies of each reference. Duplicates may come from different sources, e.g., a database and a publisher website. If so, they'll have different details. Zotero allows you to merge references, retaining the most useful information in each field. A final merged reference will keep all attachments associated with any of the duplicates. In cases where there are identical PDFs attached to the merged references, Zotero deletes the duplicate PDFs. Annotations created in Zotero are merged in the retained PDF. If you find Zotero has deleted a PDF you wish to retain, you can restore it from the Trash.
To merge references, select one of the items in center pane. Zotero will automatically also select anything it perceives as a duplicate.
In the details pane, Zotero will ask you to choose a "master item." Pick the one that has the most complete and useful details.
For fields where duplicates differ, tap the flowchart icon to the right of the field to choose which reference's details to use for that field only.
Once you've made all desired modifications, tap the Merge items banner at the top of the details pane.
A few important things to note:
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