function jsonUnique(arr, attribute){
    let new_arr = [];
    let json_arr = [];
    for(let i = 0; i < arr.length; i++){
        if(new_arr.indexOf(arr[i][attribute]) == -1){
            new_arr.push(arr[i][attribute]);
            json_arr.push(arr[i]);
        }
    }
    return json_arr;
}
console.log(jsonUnique([{a:1},{a:1},{a:2}], 'a'));