On wikipedia [1] it says "Multiple gunshot suicides are rare, but possible. In one study of 138 gunshot suicides, 5 (3.6%) involved two shots to the head, the first of which missed the brain."

I think that when Gary Webb commited suicide the CIA had already successfully disrupted his work and had no need to kill him. Example had already been given. However, was he still a threat and messing with his life wasn't an effective option, would they have killed him? They have done far worse things.

[1] https://en.wikipedia.org/wiki/Multiple_gunshot_suicide

How would he fire the second bullet?

it is also possible for a firearm to doublefire. its most common with un maintained semi auto pistols, but can happen easily enough with large calibre revolvers, due to the recoil.

It doesn't have to be unmaintained. A 1911 pistol can be made to bump fire pretty easily, especially if you have it in a weird position. While it really isn't impossible, if you take kind of a Bayesian view, the likelihood that it was mis-attributed murder probably goes up considerably once you restrict yourself to the subset of double gunshot suicides.

By not dying after the first. It happens.