No I didn’t add manually. The only plugins I have activated on my blog are: Head META Description, Optimal Title, WordPress Database Backup and now External Links.
When I create a post, it works perfectly. When I edit (no actual editing, just click on the edit link) that post and save it, all my external links have class “external external”. To rectify it, I have to delete classes of all links.
Here’s more… When I again edit the post (without deleting the classes) and save, the “external” class is added thrice (class=”external external external”). Which means, every time I click on edit and save, an extra “external” class is added.