The cave scene was one of the few genuine “unique” moments of the movie. Dark side caves should be weird.

My personal interpretation:
It’s a representation of two things,

How alone Rey is and how she feels, just stuck by herself.
How the Force can appear deterministic because it allows you to see the past and the future. Her copies make a decision behind the “real” her and then she is forced to act it out because it’s something she chose behind, then each copy in front of her has to do the same thing (snapping, waving her hand.) The future is always in motion but actions always have consequences. She moves up in sequence, trying to see the future/past to figure out who’s there, but she’s left with emptiness and just herself. The dark side can’t really help you in the end and it makes you self-centered.