What is best practice for building a mock used across several tests?

gistfile1.php

<?php
/**
 * Having trouble building the same mock in my setUp() method.
 * What is best practice for building a mock used across several tests?
 * 
 * Error:
 * 1) SystemUnderTestTest::testSecondTest
 * PHPUnit_Framework_Exception: Class "MockStaffCredential" already exists.
 */
class SystemUnderTest
{
}

class BaseCredential
{
    public function calculate($var)
    {
    }
}


class SystemUnderTestTest extends PHPUnit_Framework_TestCase
{
    protected $object;
    
    public function setUp()
    {
        $this->object = new SystemUnderTest;
        
        $this->mockPermission = $this->buildMockCredential('MockStaffCredential');
    }
    
    protected function buildMockCredential($mockClassName)
    {
        $mock = $this->getMock('BaseCredential', array(), array(), $mockClassName);

        $mock->expects($this->any())
            ->method('calculate')
            ->will($this->returnValue(true));

        return $mock;
    }
    
    //Passes just fine
    public function testFirstTest()
    {
        $this->assertTrue($mockPermission->calculate(1));
    }
    
    //Error:
    //1) SystemUnderTestTest::testSecondTest
    //PHPUnit_Framework_Exception: Class "MockStaffCredential" already exists.
    public function testSecondTest()
    {
        $this->assertTrue($mockPermission->calculate(2));
    }
    
    
}

?>

as far as i've rode, something like that:

    if(!class_exists($mockClassName)){

viewed in: http://www.sitepoint.com/forums/showthread.php?512513-PHPUnit-is-very-frustrating

Thanks