Convert PHP array from XML that contains duplicate elements

Up until now, I've been using the snippet below to convert an XML tree to an array:

$a = json_decode(json_encode((array) simplexml_load_string($xml)),1);

..however, I'm now working with an XML that has duplicate key values, so the array is breaking when it loops through the XML. For example:

<users>
    <user>x</user>
    <user>y</user>
    <user>z</user>
</users>

Is there a better method to do this that allows for duplicate Keys, or perhaps a way to add an incremented value to each key when it spits out the array, like this:

$array = array(
    users => array(
        user_1 => x,
        user_2 => y,
        user_3 => z
    )
)

I'm stumped, so any help would be very appreciated.