As you build your Zotero library, you might introduce a few duplicated items. E.g. you could have saved the same item twice from a webpage, or imported items already in your library. Fortunately, Zotero automatic identifies possible duplicates and allows you to merge them.

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

Zotero considers items to be duplicates when they share the same title.

In the “Duplicate Items” collection, select an item in the center column. Zotero will automatically co-select the other items that it thinks are duplicates. Click the “Merge n Items” button in the right column to merge the items.

You can also select a group of two or more items of the same item type anywhere in your Zotero library, right-click and select “Merge Items…” from the context menu.

Merges are automatically recognized by the word processor plugins, and don't affect your automatically generated citations and bibliographies.