Take action on first vote only
No need to hardcode a query. The voting api module provides some helper function for this.

The function you need is votingapi_select_votes($criteria = array(), $limit = 0)

The $criteria parameter takes a decent amount of options.

Here are the options:

$criteria['vote_id']  (If this is set, all other keys are skipped)
$criteria['entity_id']
$criteria['entity_type']
$criteria['value_type']
$criteria['tag']
$criteria['uid']
$criteria['vote_source']
$criteria['timestamp']   (If this is set, records with timestamps
                          GREATER THAN the set value will be selected.)


In your case you really want to pass in the 'entity_id', 'entity_type' and 'uid' to check if the user has voted in that particular content item before awarding him points.

This is the same way we prevent a user from voting multiple times in the same item with Fivestar.

Hope this helps.