There is "Duplicated items" collection in zotero that can clean items that is "save to zotero" several times in several collections. But this cleaning is post process.

I am wondering if zotero could provide an option, when turn it on, it can detect duplication when "save to zotero" from browser. It works like this: If it detect a duplication when saving, it pop up a dialog for merging item. In this way, we can make sure all items in the library are unique at any time, and cite key which generated by better bibtex is also unique too.