This question has arisen so often that it requires a special explanation:
New Game+ files do not stack. This is by design. It's not a bug. This fact has been confirmed multiple times by @Marcin Momot , and @Burza46 .Therefore, there's no option to start a second New Game+ from a completed New Game+ save. Once you finish the main quest-line in a New Game+, you cannot start over again using this same save.
In order to play again, you must restart entirely in a new regular game.
(If you kept a copy of the previous completed regular save you used to begin your first New Game+, you can reuse this save to start another New Game+. However, it must be the regular save, and not a NG+.)
I realise this can be a bit confusing, and my apologies for that, but it's how the game is setup.
Thank you. Carry on.