When a new backbone is built we must try hard to keep usage keys stable.
We use the canonical name, rank, kingdom and authorship to match to a previously existing usage, even if it was deleted before (we would then resurrect the id).
In case there are undistinguishable usages the IdGenerator fails and just creates a new identifier instead of picking one of the existing according to some rule. For example the following existing usages cause problems:
The genus Cardinalis Bonaparte and the species Cardinalis cardinalis (Linnaeus, 1758) are currently conflated and for both a new id is generated.