Does this do the job?

public function postSignup(Request $request) {

        if($request->ajax()) {

            $this->validate($request, [

                'email' => 'required|unique:users|email|max:254',
                'username' => 'required|unique:users|alpha_dash|max:16',
            ]);

            if ($this->validate->fails()) {
                return $this->validate->errors()->all();
            }
        }

        $this->validate($request, [
            'fullname' => 'required|alpha|max:20',
            'email' => 'required|unique:users|email|max:254',
            'password' => 'required|min:8',
            'username' => 'required|unique:users|alpha_dash|max:16',
        ]);
    }