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->''"Don't get me wrong, I love the voice acting in this game. But occasionally we have Chris Metzen congratulating Chris Metzen for slaying Chris Metzen."''
-->-- ''VideoGame/WorldOfWarcraft'' forums
Professional voice actors pride themselves on range. So, hiring a few good voice actors means you can take care of [[LoadsAndLoadsOfCharacters many, many characters]] with a small cast (especially if one or two actors are a ManOfAThousandVoices).
Oftentimes, this results in funny situations, like two characters played by the same person having intense conversations and heated arguments with each other. The talent is in making sure the audience doesn't know it. If jokes are made about this, it's ActorAllusion.
Sometimes this can happen in real life for technical or legal reasons where a person is required in one capacity to write themselves (in another capacity) a letter. For example, in most US states, for small corporations, a corporate officer can file an authorization to the board of directors to waive the corporation carrying workers' compensation insurance on that officer. Now, in the case of a corporation with a single owner (who therefore is both Chairman of the Board of Directors and President), the President of the corporation (who is the only officer) has to write a letter to ''himself'', as Chairman of the Board, informing himself that he (as President) is waiving the Workers' Compensation coverage on himself (the letter being sent to the Workers' Compensation Board).
In voice acting, the process is fairly simple, with the actor just doing a different take (although some good voice actors can do it in real-time). The actor's vocal range is the only thing that might betray commonality.
This is sometimes actually invoked on ''purpose'', as it can make you think, "Ohey, they're a clone? Why didn't I realize that before?".
In Live-Action this can be difficult, which requires split screen or otherwise splitting the image. This requires perfect synchronization between the different takes. Normally, the camera is stationary for this, but ''Film/BackToTheFuturePartII'' pioneered a motion controlled camera that allows for complex panning shots that have the same actor in multiple roles.
Not to be confused with InformingTheFourthWall, SoundingItOut, ThinkingOutLoud or TalkingToThemself. Compare HoldingBothSidesOfTheConversation, which is an in-universe example of this trope, where a character is pretending to hold a conversation with another non-present (or non-existent) character, in order to maintain some kind of charade.