custom EAV model saving attributes

Question

I have a custom EAV model that uses custom tables for comments that can be set on either products or categories. I have created an admin for these using the standard EAV admin form.

In my controller, I have a save action:

public function saveAction()
{
    if ($data = $this->getRequest()->getPost()) {
        try {
            $model = Mage::getModel('module/comment');
            $model->setData($data)->setId($this->getRequest()->getParam('id'));
            $model->save();

            $this->_getSession()->addSuccess(
                Mage::helper('module')->__('The comment has been saved.'));

            $this->_redirect('*/*/edit',
                array('id' => $this->getRequest()->getParam('id')));
        }
        catch (Exception $e) {
            Mage::logException($e);
            $this->_getSession()->addError(Mage::helper('module')->__('There was an error saving the comment'));
        }
    }
    $this->_redirect('*/*/index');
}

When I save the form with a new comment, the grid page shows the new comment. But, when I look in the database with the following query:

SELECT * FROM eav_comment_text WHERE entity_id = 1;

I see the following:

+----------+----------------+--------------+----------+-----------+-------------------------+
| value_id | entity_type_id | attribute_id | store_id | entity_id | value                   |
+----------+----------------+--------------+----------+-----------+-------------------------+
|        1 |             31 |          964 |        0 |         1 | test                    |
|        4 |             31 |          964 |        0 |         1 | test adding another one |
+----------+----------------+--------------+----------+-----------+-------------------------+

Can anyone explain why (a) Magento does not replace the existing value and (b) how Magento knows which one to show in the grid?