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's 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 the 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." Choose the one with 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:
This plugin works with the new version of Zotero version 7.0.7
Go to https://github.com/ChenglongMa/zoplicate/releases/tag/3.0.8
Click on zoplicate.xpi.
In Zotero, go to "Tools" and then "plugins." Then click on the tool/gear/setting icon in the upper right corner. Then click on "Install plugins from file." Once you install it, you should see "bulk merge all duplicate items" when you click on the duplicates folder.
After you have it installed you have to go to edit, settings, and zoplicate. Under master select "most detailed".
Instructions on how to use zoplicate can be found on https://github.com/ChenglongMa/zoplicate, then scroll down.