How to verify if a node in response body has unique value with postman?

The following is the response body of an API:

[
{
    "exercise_num": "1",
    "expire_date": "2019-03-11T16:31:17.935Z",
    "created_at": "2019-03-15T11:44:35.698Z"
},
{
    "exercise_num": "2",
    "expire_date": "2019-03-11T16:31:17.935Z",
    "created_at": "2019-03-15T11:44:38.363Z"
}
]

In Postman Tests, how to verify if exercise_num node in the response body above is unique?