How to ensure unique item in an array based on specific fields - mongoDB?

In my scenerio, there are authors in a collection, each author has messages and each message of author can has events. Each actor allowed to perform only one kind of action once.

db.people.ensureIndex({messages.messageEvents.eventName: 1, messages.messageEvents.actorId: 1}, {unique: true});

I added index but it has no effect. As you see below, my document has three elements which have "eventName":"vote" and "actorId":"1234" that should be against my constraint.

How to ensure unique item in messageEvents array based on eventName and actorId fields ?

Actually, i need to update the existing item without a second search and update event instead of rejecting it .

{
  "_id": "1234567",
  "authorPoint": 0,
  "messages": [
    {
      "messageId": "112",
      "messageType": "Q",
      "messagePoint": 0,
      "messageEvents": [
        {
          "eventName": "Add",
          "actorId": "1234",
          "detail": ""
        },
        {
          "eventName": "Vote",
          "actorId": "1234",
          "detail": "up"
        },
        {
          "eventName": "Vote",
          "actorId": "1234",
          "detail": "down"
        },
        {
          "eventName": "Vote",
          "actorId": "1234",
          "detail": "cork"
        }
      ]
    }
  ]
}