Should it be possible to enforce uniqueness of an object within an array within a single document using a unique index?

For instance:

db.foo.save( { a : [

] } )
db.foo.ensureIndex({_id: 1, 'a.b': 1},

);

// This succeeds. Should it fail?
db.foo.update({}, { '$push' : { 'a':

} });