The genius of Lem's underlying idea is that the duplicates, or replicants, or whatever we choose to call them, are self-conscious and seem to carry on with free will from the moment they are evoked by the planet. Rheya, for example, says, "I'm not the person I remember. I don't remember experiencing these things." And later, "I'm suicidal because that's how you remember me." In other words, Kelvin gets back not his dead wife, but a being who incorporates all he knows about his dead wife, and nothing else, and starts over from there. She has no secrets because he did not know her secrets. If she is suicidal, it is because he thought she was.
The deep irony here is that all of our relationships in the real world are exactly like that, even without the benefit of Solaris. We do not know the actual other person. What we know is the sum of everything we think we know about them. Even empathy is perhaps of no use; we think it helps us understand how other people feel, but maybe it only tells us how we would feel, if we were them.
https://www.rogerebert.com/reviews/solaris-2002
I think it is a deep mistake to correlate one's exterior appearance and behavior with a sense of selfhood. At best, it's a shallow surface imitation of the actual person. At worst, it creates a nightmarish, Uncanny Valley-esque simulacrum, not unlike Harlow's wire monkey mother surrogates. It would be better to learn how to accept and deal with loss - a reality that everyone inevitably faces.