WebTestBase::setUp() attempts to enable modules multiple times

abstract class Foo {
  public static $modules = array('foo');
}
abstract class Bar extends Foo {
}

class Baz extends Bar {
}

When running Baz, WebTestBase::setUp() would call the following:
module_enable(array('foo', 'foo', 'foo'), TRUE);
And you would get the message: Enabled modules: foo, foo, foo

Here's a fix, but it likely needs a test.