One Black Flag.

It seems like every article that has been written about Black Flag, or the other band, FLAG, states that there are "two versions of Black Flag" right now. While it is true that there are two bands going around with former members of Black Flag, playing Black Flag music, only one is Black Flag.
Black Flag, as the name would suggest, is Black Flag. They have the only consistent member, Greg Ginn. He is also the main driving force behind the band's sound. The fact that they have a new drummer and a new bass player should only be expected as Black Flag had six of each in their original run.
FLAG is some sort of revival/tribute band. It is true that four of the five of them were in Black Flag at one time, but they weren't all in the band together. They don't represent a particular line-up. They are not recording new music, and worst of all, they are not progressing the sound in any way. Progression is the trademark sound of Black Flag.
Don't get me wrong, if this seeing an odd grouping of ex-Black Flag members play early Black Flag songs without Greg Ginn is your thing, then go for it. The fact remains, however, that there is only one Black Flag.

1. Almost half of Black Flag's set is post-Damaged, while FLAG only plays a few post-Damaged songs.
2. Black Flag has progressed the sound. Even the old songs are played in a new way. FLAG sounds like they are trying to imitate the way they played in '79.
3. Black Flag is writing and recording new music. Real bands do this, revival and tribute bands typically just play old music, which is what FLAG is doing.
4. Greg Ginn is the one essential ingredient in Black Flag's original run. Sure, Keith formed the band. He also left in 1979, missing out on almost seven years of the band. Black Flag is more than Nervous Breakdown and Jealous Again. Black Flag is The Process of Weeding Out, Family Man, and In My Head.
5. Black Flag till plays instrumental songs. FLAG does not. Black Flag plays jam style songs, FLAG does not.

Black Flag was a hardcore punk band in 1979. They became so much more than that. They were experimental, improvisational, instrumental, and innovative. FLAG might do a good job of capturing 1979, but they stop there. FLAG does nothing to continue the legacy of Black Flag. Even if you don't like Rollins era Black Flag, or you don't like post-My War Black Flag, or you don't like instrumental jam band Black Flag, or you don't like 2013 Black Flag, it doesn't change the fact that there is one Black Flag.