If a document has a compound sort key (e.g.:

) and an expression tries to manipulate it, the result may only take one of the fields into account. For example, if the user tries to use {$objectToArray: {$meta: "sortKey"}}, the resulting array only has the first element (

).

I haven't encountered any cases yet, but it's possible that as we use Document/Value more we'll run into internal cases where we are manipulating Documents with duplicate fields.

In general, there is no invariant that all field names in a BSONObj are unique, but the only safe way to have a Document with duplicate field names is when you create it from a BSONObj and convert back to a BSONObj with no transformations (so that the lazy conversion behavior just returns the original BSONObj).