It appears that a mapping following this structure:
root entity -> oneToMany -> Spatial fields
will attempt to index the spatial coordinates multiple times for the same field name.
Since Spatial now uses DocValues, this is an invalid mapping.
The mapping has always been invalid as the pairs of coordinates would have been indexed in a mixed bag - so impossible to pair up a latitude with its related longitude - yet we didn't throw an exception at runtime when these were not DocValues.
We should detect an illegal cardinality for DocValue fields at boot time, and fail fast.