I am not sure if I fully understand you. But, if you merge all duplicates showed in the "Duplicate Items" in the left pane, then you have no duplicates in any collection/subcollection. You can see one item in different collection/subcollection, but this is still one record in your Zotero database.
Hmm I'm working under a Group Library. There is no 'Duplicate Items' in the left pane for my group library but I do see it for 'My Library'. If I right click on My Group Library named 'ESRM', I can select 'Show Duplicates'. However, this function is not present for any collections I have created under the group library.
Sorry, I am using version 5.0 beta and I see 'Duplicate Items' in group libraries too. But that is not important due to the princip is still the same. If you merge all duplicities in the 'root' collection of your group library, then you can not have any duplicity in subcollections.
Sure, but if, for whatever reason, you just want to see duplicates in a given collection/subcollection, there is indeed no way to do so.
Would be difficult to do, too--should that show just items that are in that collection twice, or any duplicate that has at least one copy in that collection?
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Would be difficult to do, too--should that show just items that are in that collection twice, or any duplicate that has at least one copy in that collection?