In general, when you add a citation to a document we check it against the existing references in the document and don’t allow you to add duplicates.
However, in some cases we have limited or outdated knowledge about which references are in the document, and this can result in duplicates:
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Copying references from another document. When you paste text and references from another document, we are not immediately aware of it - we only become aware of it when you format citations, at which point we retrieve the metadata. This may result in duplicates because we make no prioritization among recognized and unrecognized references when formatting the document.
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Adding references to a copy of a document before it has been formatted. This is similar to the above case. When you make a copy of a document, we are not aware that it contains citations (since we don’t want to process every document you open). It is therefore possible to add duplicates of citations already in the document. From our point of view, it seems like you are writing a new document, then when you format citations, like you copied in all the paragraphs/references from the old document, leading to the same problem. This second cause can be avoided by always formatting a document when you make a copy.
Note that the common use case of colleagues working on the same document does not result in duplicates.
When stuck with duplicates this may be the easiest solution:
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Go to the view all references page. Locate the duplicate you want to remove and edit its metadata to something which will clearly identify it (i.e. change the year to 9999).
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Format the citations in the document.
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Remove all copies of the duplicate citation from the text (manually).
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Remove the undesired copy of that citation from the (“View all references”) page.
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Re-insert the removed citations as normal.