postal.js/src/DistinctPredicate.js

var DistinctPredicate = function() {
    var previous;
    return function(data) {
        var eq = false;
        if(_.isString(data)) {
            eq = data === previous;
            previous = data;
        }
        else {
            eq = _.isEqual(data, previous);
            previous = _.clone(data);
        }
        return !eq;
    };
};