Zotero can help you identify duplicated items and allow you to merge them together.
You can find directions on Finding Duplicates and Merging Duplicates from Zotero's Duplicate Detection page.
Zotero suggests you always resolve duplicates by merging instead of deleting one of the duplicates. This allows you to retain the data from the collections and tags of merged items.
Please note, Duplicate Detection only works within a library. It does not detect duplicates in different group items.