PHP Error after enabling Twitter API

Hi,
I activated the following modules:
1. Autoload 6.x-2.1
2. OAuth 6.x-3.0-beta4
3. Twitter 6.x-5.0

Steps 1 + 2 are working fine. After step 3 I get the following PHP error on every page load:

Fatal error: Cannot redeclare class OAuthException in /www/htdocs/w00a5a9f/www/sandpit2/sites/all/modules/testing/oauth/lib/OAuth.php on line 8

I have also the "Facebook for Drupal" module enabled.
Any idea, what's going wrong?

Thx for your help!