Lena's tattoo
Spoilers
I think this, beyond the eyes, is proof that the Lena that returned is a copy. She has an infinity tattoo on her forearm when she returns. I re-watched the film and she does not have the tattoo before entering the Shimmer. Another character, Anya, the one that goes nuts, does have this tattoo on her forearm though. Also, when Lena is standing with the duplicate and holding the grenade you can see the duplicate has this tattoo. The story she is telling is not the truth, the original Lena died, probably on the door. The duplicate used the incendiary grenades to destroy the lighthouse. Why? Who knows, beyond giving a plausible cover story it doesn't make much sense.