Error when running watch with assets-versioning

[YoussefTaghlabi]

I have the following grunt configuration:

grunt.initConfig({

    less: { 
        dev: { 
            files:[ {
                expand: true,   
                cwd: 'static/less/',
                src: ['*/master/**/*.less', '*/views/**/*.less'],
                ext: '.css',    
                dest: 'static/csscache/'
            }]
        }
    },

    assets_versioning: {
        options: {
            tag: 'hash',
            hashLength: 32
        },

        css: {
            options: {
                post: true,
                //versionsMapTrimPath: "static/csscache/",
                versionsMapFile : 'staticmap.json',
                versionsMapTemplate: 'json.tpl',
                tasks: ['less:dev']
            }
        }
    },

    watch:{
            less:{
                files: "static/less/**/*.less",
                tasks: ['assets_versioning'],
                options: {
                    nospawn: true
                }
            }
    }
});

When I run "grunt watch" and touches one of the less files, all less files get recompiled and versioned.
However, when I touch one of the less files a second time, I get the following and nothing gets versioned:
"
Running "assets_versioning:css" (assets_versioning) task
Versioning files from less:dev task.
Versioning files from assets_versioning:css task.
Warning: Task 'assets_versioning:css_post_assets_versioning' already exists!
"

Looking at "tasks\helpers\task.js", it seems that when a grunt.config key already exists, it fails in purpose:

Task.prototype.createSurrogate = function (filesObj) {
...
if (grunt.config(surrogateTaskConfigKey)) {
grunt.fail.warn("Task '" + surrogateTask + "' already exists!");
}
...
};

Task.prototype.createPostVersioningTask = function (filesObj) {
...
if (grunt.config(taskConfigKey)) {
grunt.fail.warn("Task '" + postTask + "' already exists!");
}
...
};

Any reason it's done this way?