Hello @kartik,
Currently you have to use two streams for each dest when using file.contents as a stream. This will probably be fixed in the future.
var gulp       = require('gulp');
var rename     = require('gulp-rename');
var streamify  = require('gulp-streamify');
var uglify     = require('gulp-uglify');
var source     = require('vinyl-source-stream');
var browserify = require('browserify');
var es         = require('event-stream');
gulp.task('scripts', function () {
    var normal = browserify('./src/index.js').bundle()
        .pipe(source('bundle.js'))
        .pipe(gulp.dest('./dist'));
    var min = browserify('./src/index.js').bundle()
        .pipe(rename('bundle.min.js'))
        .pipe(streamify(uglify())
        .pipe(gulp.dest('./dist'));
    return es.concat(normal, min);
});
Hope it helps!!
Thank You!!