[{_id:"1","field":[{"name":"A"},{"name":"B"}]},{_id:"2","field":[{"name":"C"},{"name":"A"}]},{_id:"3","field":[{"name":"E"},{"name":"F"}]},{_id:"4","field":[{"name":"G"},{"name":"H"}]}]

db.collection.aggregate([{$unwind:"$field"},{$group:{_id:"$field.name",field:{$first:"$field"}}},{$replaceRoot:{newRoot:"$field"}}])