Some operations, like remapping and ID (Object) to another, can lead to
having the same object in more than one base.
While this is not a valid state, this is being taken care of by the
`BKE_layer_collection_sync` call, so the object-to-base GHash generation
itself should be resilient to such issue.
Note: another way to fix this would be to make remapping post-process
code check explicitely for such doublons, but I would rather avoid
adding even more 'specialized' code there, it already has to deal with
too many of those corner cases.
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