NikonA new firmware of D6 has been released a few days ago, and the version number of the firmware is 1.33. Nikon will pay more attention to the flagship model of the DSLR and will not allow bug, which is designed to fix the small bug.
The description of the new firmware reads as follows: "solves the problem that in a few cases, pressing the shutter release button will cause two consecutive photos to be taken when the single frame release mode is selected." To put it simply, it fixed a small mistake that a single beat may become a double beat.
If you are a D6 user, please update it as soon as possible. The portal is here.