Add non existing element to array using dynamoose

I have a table in AWS DynamoDB, using dynamoose for querying from NodeJS.
An item structure is like:

{
    name: 'x',
    gmail: 'x@gmail.com',
    mob: '987654321',
    Friends: [ {name:'a'}, {name:'b'} ]
}

I need to update the array 'Friends' such that when adding a new array of friends like below

[ {name:'a'}, {name:'b'}, {name:'c'} ]

it should add non existent element only!