I didn't put a whole lot of explanation into my second episode, because I figured that the characters would be too confused and shocked by everything that was going on to have a clear speculative conversation. What's going on is that the Fred, Astras etc. on the Dead Earth are part of a Convergence spell, whose maker and purpose we as of yet do not know. It draws in three Freds, three Astras, and three wild cards. (Wincerind is not strictly speaking part of the Convergence but got dragged along.) Whenever one of these dies--or chooses to leave, once they find the one who created the spell--an equivalent is snatched from another world to take their place. I'd suggest having the new Astra be called Astra 3-2, and so on for future Astras and Freds in the Convergence, except for the ones who have changed to the extent that they've taken different names.