You can delete duplicates with plugin named 'Duplicates Merger for Zotero', or through looking into 'My library-duplicates items' then click items to merge.
I had over 7000 duplicates after importing a Mendeley library into Zotero and the Duplicates Merger saved me SO MUCH TIME!! I highly recommend it. I still had to monitor its progress because it stopped every time the duplicates had different item types, but it was still infinitely better than manually clicking them one at a time.
https://github.com/frangoud/ZoteroDuplicatesMerger