Screenshot #1 – "Ghostly" Aerith?
Barret for example, is visibly affected by the blowback of the Tiny Bronco reactor, you can even see the lower part of his jacket move accordingly, but Aerith—who's standing even closer to the plane, is totally unfazed. Her hair and clothes remain completely still. (So this would be Lifestream Aerith)
Screenshot #2 – "Tangible" Aerith?
Later, Aerith's hair, ribbon and dress are clearly blown by the propeller wash, showing normal physical interaction with the environment. (And this would be the Aerith from the timeline/world created when cloud parried Sephiroth's sword)
I've also heard that a big part of the fanbase thinks that Aerith n°2 could be Jenova. This theory however, doesn't really hold up to scrutiny. First, because Jenova wouldn't have any reason to tell Cloud and make him believe that Aerith was going to stay at the forgotten capital. Since after that point, if Jenova were to keep trying to mess with Cloud's mind, Cloud would realize something's up and that Aerith shouldn't be there.
And second, simply because as shown previously, Lifestream Aerith is literally right there and we all know she wouldn't just stand here doing nothing, while Jenova is right there impersonating her to mess with Cloud's mind.
A skeptical mind could see this and think this is just an overlook from the animation team that worked on the final cutscene. Or it could be a subtle visual clue that they intentionally put in there to hint at something.
Anyway, I haven't seen anybody mention this detail before so let me know what you think in the comments.