(Sort of AU since I like the idea of Riku Replica, Vanitas, and Naminé as a trio)

Timeline: After Kingdom Hearts III

Despite the fact that the world already has Riku, after Naminé was brought back with her own vessel, Riku got another vessel for Riku Replica to be brought back. As the scientist placed Riku Replica's heart inside the vessel, Naminé and Riku were pretty excited.

So, Riku Replica soon woke up in his own body, and Riku was right there by his side, happy that Riku Replica could now be his own person.