Angularjs one time binding is called twice

var app = angular.module('myApp', []);

app.controller('myCtrl', function($scope) {
    $scope.foo = function() {
        console.log('foo'); 
        return 'foo';
    }
});

<!DOCTYPE html>
<html>
    <script src="https://ajax.googleapis.com/ajax/libs/angularjs/1.5.5/angular.min.js"></script>
    <body>
        <div ng-app="myApp" ng-controller="myCtrl">
            <span ng-bind="::foo()"></span>
        </div>
    </body>
</html>