As described in issue #56733, there is a possibility of a race condition in SSmapping.add_new_zlevel - either during Z level incrementing, or the proc itself, as it only expands the amount of registered z-levels after being finished.
While the first hopefully shouldn't happen, it can still manifest because of the CHECK_TICK within. A practical result is that two concurrent calls to this proc will both create a new Z-level, but actually report having made the same first one.
This is problematic for map_templates that will then load blindly to the reported Z-level when using load_new_z, overwriting each other. We sometimes encounter that end result on CM13 codebase because of a map load that can be triggered by an user topic - if two people click at the same time, it's not unlikely for the first call to sleep in CHECK_TICK and let the second run, causing double-loading.
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