x = [{"id":"item1","val":"Items"},{"id":"item1","val":"Items"},{"id":"item1","val":"Items"}]

var clickId = // could be "item1", "item2"....
var found = $.inArray(clickId, x);  //
if(found >=0)
{
    x.splice(found,1);
}
else{
    x.push(new Item(clickId, obj)); //push json object
}

x = [
    {"id":"item1","val":"Items"},
    {"id":"item1","val":"Items"},
    {"id":"item2","val":"Items"}
];

var clickId = [];
var list = JSON.parse(x);
$.each(list, function(index, value){
    if(clickId.indexOf(value.id) === -1){
        clickId.push(value.id);
    }
});