Laravel 5.2: Your requirements could not be resolved to an installable set of packages(duplicate) - json

Question

I think question is almost same but error is different.
I just install laravel 5.2 and trying to install quickadmin packages. But I got this error. I also update composer. I tried this suggestion and also some other suggestions. But It's not working. I also tried older version of composer.json configuration. But unfortunately It's not working.
Error:
Here is my Composer.json configuration without changing anything.
{
"name": "laravel/laravel",
"description": "The Laravel Framework.",
"keywords": ["framework", "laravel"],
"license": "MIT",
"type": "project",
"require": {
"php": ">=5.5.9",
"laravel/framework": "5.2.*"
},
"require-dev": {
"fzaninotto/faker": "~1.4",
"mockery/mockery": "0.9.*",
"phpunit/phpunit": "~4.0",
"symfony/css-selector": "2.8.*|3.0.*",
"symfony/dom-crawler": "2.8.*|3.0.*"
},
"autoload": {
"classmap": [
"database"
],
"psr-4": {
"App\\": "app/"
}
},
"autoload-dev": {
"classmap": [
"tests/TestCase.php"
]
},
"scripts": {
"post-root-package-install": [
"php -r \"copy('.env.example', '.env');\""
],
"post-create-project-cmd": [
"php artisan key:generate"
],
"post-install-cmd": [
"Illuminate\\Foundation\\ComposerScripts::postInstall",
"php artisan optimize"
],
"post-update-cmd": [
"Illuminate\\Foundation\\ComposerScripts::postUpdate",
"php artisan optimize"
]
},
"config": {
"preferred-install": "dist"
}
}
Any suggestions how to solve in laravel 5.2?