Do I need to run Captcha and Mollom simultaneously?
If you're satisfied with the Mollom module, then I don't think it makes sense to also leave the Captcha module enabled/installed.

However, if you're looking for some kind of extra protection/layer but without those double captcha forms, then you may want to consider replacing Captcha with the Honeypot module. Because of the technique used by Honeypot, you don't risk to have double captcha forms in that case.

Some more details about the Honeypot module (from its project page):


  ... uses both the honeypot and timestamp methods of deterring spam bots from completing forms on your Drupal site (read more here). These methods are effective against many spam bots, and are not as intrusive as CAPTCHAs or other methods which Punish the user.


Note: even if you don't want to consider using the Honeypot module (for whatever reason), keep "the message" in mind about what that link quoted from its project page (Punish the user) is trying to explain whenever your researching similar modules.