Great first game jam! I think you nailed the core mechanic, and I love the fact that you can jump on your doppelganger for more height. Something about pressing the rewind button and then jumping into a pit just feels super satisfying to do. My only negative criticisms would be the details on your collision detection and character movement. The character can seem like it twitches sometimes, and the hitboxes are a little bit off from the visuals. It also seems like you stick to corners of platforms a bit if you're pressing against them.
Great game though!