To remove duplicate records--also called deduping--open your Zotero 6.0 desktop software.
1. Click on the duplicate items folder. This will display a listing of items Zotero believes to be duplicates.
2. In the middle pane, click on a duplicate item.
3. Select which version you would prefer to keep; differences will be displayed beneath the listing of the different versions. Once you've decided on a final version, click on the "Merge Items" button and Zotero will remove the duplicate reference(s).