Jakub Kicinski [off-list ref] writes:

I think the description fields need to either be concatenated, or an
array.

Yep.

https://www.rfc-editor.org/rfc/rfc8259#section-4 :

   The names within an object SHOULD be unique.

   An object whose names are all unique is interoperable in the sense
   that all software implementations receiving that object will agree on
   the name-value mappings.  When the names within an object are not
   unique, the behavior of software that receives such an object is
   unpredictable.  Many implementations report the last name/value pair
   only.  Other implementations report an error or fail to parse the
   object, and some implementations report all of the name/value pairs,
   including duplicates.