I discovered this naturally in my friend’s survival realm and then tested it out on a creative singleplayer world. From my understanding, this only affects Phantoms.

Steps to Reproduce (Survival):
1. Spawn a phantom using the insomnia mechanic.
2. Place a boat. For best results, place it on top of a two block pillar.
3. Wait for phantom to swoop and get behind the boat so it gets caught.
4. Get close to the phantom. Whether that be breaking the pillar so the boat falls, building up to it, doesn’t matter. This step doesn’t affect the results, only makes it easier to see.
5. Look away. Not too far, only so that the phantom is just barely out of view. For best results, look up.

Steps to Reproduce (Creative):
1. Get a phantom spawn egg, and either a boat (a) or blocks (b). For best results, choose barrier blocks.
2a. Place the boat and a block next to it. Spawn the phantom against the block so it automatically goes into the boat.
2b. Place blocks to make a small box, preferably 1x1 of space. Make the walls about 2-3 blocks tall for easier trapping. Spawn the phantom inside and quickly cover up the opening.
4. Get close to the phantom. This step doesn’t affect the results, only makes it easier to see.
5. Look away. Not too far, only so that the phantom is just barely out of view. For best results, look up.

Observed Results:
What appears to be a clone will show up right above the original phantom. The similarities it has with the original is that its wings still give off trail particles, it shares the overlay upon being hurt or set on fire, and it plays the normal death animation. The differences is that its wings do not flap and it does not give off fire or potion particles.

Expected Results:
…Normally phantoms don’t clone themselves?

Additional Notes:
The clones appear from any distance. You could run 50 blocks away and it would be there as long as the original isn’t. If you fully face away and go in F5 you may see it briefly appear before vanishing. If the blocks you trap the phantom in are solid, the clone will appear submerged. If the phantom is named, the clone will not be.