(Why didn't bug 5977 come up? Because when you report a bug, the list of possible duplicates shows only bugs reported under the same project. This makes sense for most projects, but Launchpad (ab)uses multiple "projects" to cover the single Launchpad codebase -- for example, bugs about the Launchpad bug tracker go under not "launchpad", but "malone". (Yes, really.) Therefore, they don't appear when you report bugs under "launchpad".)
Ok, revising ...
(Why didn't bug 5977 come up? Because when you report a bug, the list of possible duplicates shows only bugs reported under the same project. This makes sense for most projects, but Launchpad (ab)uses multiple "projects" to cover the single Launchpad codebase -- for example, bugs about the Launchpad bug tracker go under not "launchpad", but "malone". (Yes, really.) Therefore, they don't appear when you report bugs under "launchpad".)