Collection name uniqueness for children of the same parent not enforced
Collections belonging to the same parent should have unique names within the group. This is presently not checked.
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Personally, I can make sure that such a collision does not happen in my library. However, I would think that people intuitively expect that child collection names uniqueness is also controlled by the system.
It's not just about the technical effort — it's about the many new problems it would pointlessly create for users. It would mean that, if you dragged a collection to another collection, Zotero would need to prompt you to do something complicated and error-prone — replace the collection (losing data), merge items (and subcollections? recursively? what if those had the same name too? what if the set of items in each had important differences that were lost?), or cancel the operation (leaving the collection in a more difficult place in your library to resolve the problem). It would mean that if you and someone else in a library created a collection with the same name, you would get a conflict that was essentially impossible to show a conflict resolution window for, so it would have to automatically rename the collection and then sync that back to the other computers, possibly creating more conflicts.
It's just not necessary to create these and other obstacles for something entirely trivial to fix manually (which is presumably why many library-based (and even folder-based) programs don't enforce this, by design).