If you have imported references on the same subject from more than one online source, you may find that you have some duplicate references in your library. To keep your library organised, it is important to ensure you have removed any duplicate references. Zotero has a dedicated folder for such items.
In the Duplicate Items folder, you will see a list of items that have been identified as duplicates, if there are any. When you select an item, the relevant duplicates will be highlighted. You will see their records in the right-hand side pane. Choose the version with the most information and keep it as the master version. You can then merge the records by clicking the Merge items button.
Select an Item in the Duplicates folder.
Duplicates will be highlighted.
Check the records of each duplicate on the right-hand side pane.
Choose the version which has the most information and use it as the master item.
Click the Merge items button.
De-duplicate after each Zotero session. This will ensure that you are not using a reference you have tidied up on one page of your document and the same reference, with missing information, on another page of your document.