Determine if customer is new in customer_save_after

First of all you can define your observer as singleton for both events

<global>
    <events>
        <customer_save_after>
            <observers>
                <namespace_module>
                    <type>singleton</type>
                    <class>namespace_module/observer</class>
                    <method>doSomething</method>
                </namespace_module>
            </observers>
        </customer_save_after>
        <customer_register_success>
            <observers>
                <namespace_module>
                    <type>singleton</type>
                    <class>namespace_module/observer</class>
                    <method>doSomething</method>
                </namespace_module>
            </observers>
        </customer_register_success>
    </events>
</global>

In this case the same observer object will be used for both events. So you can create flag in your observer and before doing some actions check it.

class [Namespace]_[Module]_Model_Observer
{
    protected $canDoSomething = false;

    public function doSomething($observer)
    {
        if (!$this->canDoSomething) {

            //here your code

            //now set flag to true to prevent executing your code twice 
            $this->canDoSomething = true;
        }
    }
}

Now your code will be executed only once.