Duplicate nodes created when using parallel importer with 'Retain Content IDs'

Affected version
v2.1.10 Patch 2, XFI 1.3.2


When using the parallel importer with 'Retain Content IDs' enabled, multiple 'Imported Content' (or e.g. blog parent) nodes can be created.

I believe this is because each child process loads its own copy of the import session in memory, checks it for an importContainerParentNodeId (or blogUsersParentNodeId), finds nothing and so creates a new node. Because the session is not shared (as far as I can tell?) every child process does this independently.