Gulp - copy and rename a file

I'm extremely new to Gulp. I'm basically trying to watch for a modified JavaScript file, and then make a new copy of it with a new name. (eventually there'll be some processing on it, but Rome wasn't built in a day).

My (naive) attempt is this:

gulp.task('default', function() {

    return gulp.watch('../**/**.js', function(obj){
        gulp.src(obj.path)
            .pipe(gulp.dest('foobar.js'));
    });

});

This takes the modified file and successfully copies it into a folder now called foobar.js. Is there anything simple I can replace gulp.dest('foobar.js') with that will simply copy and rename the src file in place?

---

EDIT

By copy in place, I mean I want to take the modified file, and make a copy of it right where it currently is with a new name. The equivalent of clicking the file (in windows) and hitting control-c control-v, then renaming the resulting file.

I've come up with this

gulp.task('default', function() {
    return gulp.watch('../**/**.js', function(obj){
        if (obj.type === 'changed') {
            gulp.src(obj.path, { base: './' })
                .pipe(rename(function (path) {
                    path.basename = path.basename + 'modified';
                }))
                .pipe(gulp.dest('./'));
        }
    });
});

It's very, very close, but it only works once. The second time I modify the (same) file, I wind up with blahmodifiedmodified, the next time I get blahmodifiedmodifiedmodified and so on.

Is it refusing to overwrite an existing file, or is something else wrong?