日志含义SQLSTATE[23000]:完整性约束违反错误

在注册新电子邮件时，我可以成功注册新用户，但是当我试图使validation rule限制现有电子邮件注册时，而不是给出验证错误消息，它给我Integrity constraint violation error，

这是我的controller

 public function postRegister(Request $request){
        $v = validator::make($request->all(), [
            'first_name' => 'required|max:30',
            'last_name' => 'required|max:30',
            'email' => 'required|unique:users|email',
            'mobile' => 'required|min:10',
            'password' => 'required|min:8',
            'confirm_password' => 'required|same:password'
        ]);
        if ($v->fails())
        {
            return redirect()->back()
                ->withErrors($v->errors())
                ->withInput(Input::except('password', 'confirm_password'));
        }

        else{
            // validation successful ---------------------------
            /*
            add data to database
             */

}

这是我的migration

public function up()
    {
        Schema::create('users', function (Blueprint $table) {
            $table->increments('id');
            $table->string('first_name', 80);
            $table->string('last_name', 80);
            $table->string('email')->unique();
            $table->string('mobile',80);
            $table->string('password', 60);
            $table->boolean('confirmed')->default(0);
            $table->string('confirmation_code')->nullable();
            $table->float('amount', 80);
            $table->rememberToken();
            $table->timestamps();
        });
    } 

如何修复这个错误?