Keywords allow the user to open a bookmark without navigating the bookmark hierarchy. However, an artificial constraint forbids to assign the same keyword to multiple bookmarks.
There is no fundamental reason why a keyword should be unique.
Here is the use case:
I want to open several bookmarks that I always want to have access to. Currently it is possible to open several bookmarks by left-clicking the bookmark folder.
When re-using the same keyword for several bookmarks, all those bookmarks can be opened at the same time.
Worse: re-using a keyword silently removes the assignment to a previous keyword!
This is a big NO-NO! The user should at least be warned!
This keyword feature is beginning to smell like a beta feature.
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An alternative is also to allow to assign a keyword to a bookmark folder.
That's a great idea. Entering a non-unique keyword into the URL should return those matched bookmarks in the URL returned dropdown - that the user can then select from.