Before writing, you should remove duplicates from your Zotero library, otherwise you may have the same reference in your bibliography more than once if you have referred to several different versions of the same reference in the text.
- Click on the Duplicate Items button on the left sidebar.
- Zotero will list the references from your library that it interprets as duplicates.
- Clicking on a reference in the right-hand margin opens a menu where you can choose which reference to keep as the base when merging a reference.
- Select Merge / Merge items.
- After deleting the duplicates, the merged reference file created from them will still remain in all the folders where the references were before the duplicates were deleted.
- It is also possible to merge tuples manually. If you find two of the same reference, you can select both by holding down the CTRL key. Then right-click on the menu that opens and select Merge Items to merge the references.