Nothing inherently "wrong" with him, he is just wrong for JLA and The Flash film. Particularly the first films in general. His very presence doesn't require just a simple explanation like "well there's more than one Green Lantern" like subbing Stewart for Jordan. There was always more than ONE Green Lantern from the very start (not talking about Alan Scott), it's one of the mainstays GL was founded on. The Flash on the other hand was created with the intention that there is only ONE Flash. There's no "Flash Corps", they do not need a Flash Corps, there doesn't need to be twin Flashes or several Flashes running around (from the same dimension (and timeline/era at least)). That's stupid and unnecessary, it defeats the purpose. It also takes away from The Flash's uniqueness and his place in the DCU; this guy got his powers from an unrepeatable one-in-a-million accident, it pushes plausibility that it even happened once, but to say it happened twice, that loses all believability, IMO. It was a hard sell for even in the comic books, and that's comic books. This is movies we're talking about, a much, much harder sell. And unlike Avengers (what with the aliens and all), DC hasn't even created anything to build on to introduce the more "silly" or "fantastic" or even "implausible" elements of their Universe yet (re: Iron Man, Agent Coulson, Fury, S.H.I.E.L.D, etc). But when they do it will be something that has to be done gradually like they did with Iron Man. The first Iron Man he is in Afghanistan, it's very real world grounded. By the time we get to Iron Man 3 there's been Avengers, aliens, "magic", etc, and the audience is ready to buy it and make the jump, but that's only because they were inched just a little bit closer bit by bit with each Marvel (or even Iron Man) movie. They'd have never bought or believed it if they were hit with it all at once right from the get go.
The only way you get two Flashes, especially two at the same time or back to back (a 1000 year gap MAY be acceptable...maybe) is that they get super speed in different ways. Which if you do that you're going to get a lot of hyperbole from people who are like "Damn, you can get super speed almost any way! Nothing special about that. Any freak accident = super speed in a DC comic film!" It's just an all around bad idea. Like I said, The Flash is one of the few DC characters to have superpowers from an accident, that has always been a mainstay of the character, something they founded his creation on like how GL has the GL Corps, that it was a freak one-in-a-million unrepeatable chance - in fact, The Flash is the only MAJOR DC character out of the big 7 to have his powers from an accident. How would it seem if you had multiple guys walking around with radioactive spider bites? That'd just be kind of stupid. And I'm not knocking Wally West because I like him as Kid Flash and there's a great idea for a Teen Titans movie but for the foreseeable, immediate future, Wally West has no place in the DC movie universe, certainly not in the initial Flash film and JLA. Even if he showed up he'd be in his damn teens (otherwise you'd have to make Barry Allen really old, like Jay Garrick), and nobody wants to see that. I would not buy him as The Flash or take him seriously as a respectable superhero holding his own with the JLA. No, they need THE Flash for these movies.
Create the language, then define the vocabulary.