phpunit with dbunit: how can i keep data in my db across tests?

i have a phpunit question regarding dbunit and how to keep data created in the database by one test for use in the next. i'm new to phpunit (we've been using an in-house tester for years but are finally trying to get with the modern age), so i apologize if this is a trivial issue.

the desired effect i have a mysql table that contains a column that is a unique key. if an attempt is made to insert a duplicate of this column, special things happen that i would like to be able to test. i have written a test to insert a value into this column (and test its success) and then written another test immediately afterwards to test how the class fails on attempting a duplicate value. i'd like to be able to catch that exception and test it. i am using dbunit to pre-fill my db with all the pre-filly stuff i need.

the problem at the commencement of each test it appears as if getDataSet() is called and, as a result, the unique key data i insert in the first test is no longer there to test against. consequently, i can't test the anticipated failure of inserting duplicate unique keys.

what i'm looking for well, obviously some way to persist the database data across tests; avoid calling getDataSet(), perhaps, at the beginning of the second test.

i certainly hope this is possible. i can't imagine why it wouldn't be; it seems like people should want to test duplicate insert! i am willing to entertain other solutions if they accomplish the task.

thanks in advance!

here's my test, stripped down to the relevant bits.

<?php
class UserPOSTTest extends \PHPUnit_Extensions_Database_TestCase
{

    static private $pdo = null;
    private $conn = null;

    /**
     * @return PHPUnit_Extensions_Database_DB_IDatabaseConnection
     */
    public function getConnection()
    {
        if($this->conn === null) {
            if (self::$pdo == null) {
                self::$pdo = new \PDO('mysql:host=localhost;dbname=thedatabase', 'user', '*********');
            }
            $this->conn = $this->createDefaultDBConnection(self::$pdo, "db");
        }
        return $this->conn;
    }

    /**
     * @return PHPUnit_Extensions_Database_DataSet_IDataSet
     */
    public function getDataSet()
    {
        // this is returned at the beginning of every test
        return $this->createFlatXmlDataSet(dirname(__FILE__) . '/some_data_set.xml');
    }

    /**
     * test the insertion of the value "unique key value" into a column set as UNIQUE KEY in mysql
     * since getDataSet() has cleared this table, it passes.
     */
    public function uniqueKeyTest_passes() 
    {
        $inserter = new Inserter("unique key value");

        $this->assertEquals($inserter->one,1); // just some bogus assertion 

    } // uniqueKeyTest_passes

    /**
     * run the exact same insert as in uniqueKeyTest_passes() above. the purpose of this test is to
     * confirm how the Inserter class fails on the attempt to insert duplicate data into a UNIQUE KEY column.
     * however, the data inserted in uniqueKeyTest_passes() has been scrubbed out by getDataSet()
     * this is the crux of my question
     */
    public function uniqueKeyTest_should_fail() 
    {
        try {
            // exact same insert as above, should fail as duplicate
            $inserter = new Inserter("unique key value");
        }
        catch(Exception $e) {
            // if an exception is thrown, that's a pass
            return;
        }

        // the insert succeeds when it should not
        $this->fail("there should be an exception for attempting insert of unique key value here");

    } // uniqueKeyTest_should_fail 

}