[{posts:[{post:{id:"dkdkd",title:"hello",body:"hello world",tags:[1,2,3]}},{post:{id:"ccc",title:"hello2",body:"hello world2",tags:[3,4,5]}}]}]

db.collection.aggregate([{$project:{_id:0,tags:{$reduce:{input:"$posts.post.tags",initialValue:[],in:{"$setUnion":["$$value","$$this"]}}}}}])