Eddie Murphy in the Nutty Professor, Michael J. Fox in Back to the Future 2, Mike Myers in Austin Powers. These are some of the most notable performances of actors playing 2 or more roles in a movie. Should the actors take the credit for their repeated and consistent portrayals of multiple characters or should the editors and visual effects team claim the accolades for their tireless efforts?
The actor's ability to portray a multitude of characters not only displays their skill, but showcases their total understanding of how scenes are developed and finished in post-production. In many cases, the actor will be the only person in front of the camera speaking to an empty marker. To keep an image of a living, breathing, communicating being were one doesn't appear may look like an asylum patient rambling to themselves. Thinking of the way you would speak to another and notice as they shift from side to side while they react to the words you are speaking. The difficulty is not imagining a human form sitting across from the actor, rather the treatment of the form as a dynamic object presents some acting skill. While this is the simplest method of 'cloning', this requires double the amount of filming time as well as time consuming post production cut sequences not to mention the abilities of the actor.
The responsibilities of making the actors efforts appear as seamless and natural as possible lays solely with the editors. Lower budget shows may not rely on CGI technology to create a new character or clean up live action mistakes. All the work that is done in front of the camera must be layered and finished to the point where it is believable that the actor is speaking with their double. Other methods exist where the actor will play their part with a stand in wearing a green mask. The beauty in the technology to allow film makers to superimpose another actor's face onto a blank canvas has reduced on-site production time and also assisting the actors responses to real time actions, this method can be compared more closely with animation than acting. However, without their timing and technical skill the acting will have been in vain and the scene would be awful.
There’s no question that TV shows and movies with actors copy and pasted in the same scene contributes to the mood of the piece. It can add a layer of humor or strictly focus on inner turmoil depending on the scene. I believe that shows, such as Orphan Black, rely on teamwork between production staff and actors to complete the final piece. So too, should credit be shared between the contributors.