SOURCE

var arr = [{
   key: 'AUDIT_RATING',
   value: 'AAA'
  }, {
   key: 'AUDIT_RATING_RES',
   value: 'BBB'
  }, {
   key: 'AUDIT_RATING',
   value: 'CCC'
  },
];


function setUnique(arr) {
   var result = [];
   var obj = {};
    for(var i =0; i<arr.length; i++){
     obj[arr[i].key] = arr[i].value;
    }
    for (let key in obj) {
        result.push({
            key:key,
            value:obj[key]
        })
    }
    return result
}



    console.log(setUnique(arr))