Good question. I would guess it's very low if we're talking exact match. It would have to match N features. There's effectively an infinite number of combinations, many more than the 8 billion people curently living.
In reality, there are already real-world doppelgangers, where a typical observer would confuse two (unrelated) individuals. I'm guessing that could happen with a synthetic image too, but it's probably not any worse than a real-world matching error. (The bigger concern is the related one of deepfakes, which don't need any kind of synthesis, they just act directly on the real person's image.)
In reality, there are already real-world doppelgangers, where a typical observer would confuse two (unrelated) individuals. I'm guessing that could happen with a synthetic image too, but it's probably not any worse than a real-world matching error. (The bigger concern is the related one of deepfakes, which don't need any kind of synthesis, they just act directly on the real person's image.)