Every frame the inputs of the player are saved in a list, and 60 frames (*) later the Shadow copies that input, mirrored. They actually both use the same object/class, and the code for player or the code for shadow seperately are very small. The only difference is the spells they cast and the way the input happens (from key reads for the player, from the list reads for the shadow).
When shooting the bullet remembers who cast it so you can't self-damage, but that also is just one unique object. There's no shadow-bullet or player-bullet. I'm pretty proud of how few code I had to reuse.
(*) If the Shadow uses level 3 time this amount gets lower, making the Shadow copy the player faster