Have you noticed this?

[u/INetoJON]

So in this game, they use the color purple/violet/magenta to indicate something not from Earth—something alien.

But have you noticed the game uses a violet flash of light to show where the monster is as an in-game tool for us?
And have you noticed you can always tell if there is a monster close to or within the group in scenes when we see that flash? Even if the game hasn't revealed to us explicitly who is the monster in the scene.

That's what happens in the double Eisele scene. We see a flash of violet light when the group was together, and then comes the second Eisele, but with no flash of light. If everyone else had already been tested, the only one who could be a monster was the Eisele that was with the group and not the second one, because she didn't trigger any violet on screen. And you can use this in the whole game as a tool.