Java Edition:Multiple Ender Dragons
Normally, a Minecraft world is supposed to only have one ender dragon at a time. However, there are ways to trick the game into summoning more than one, and one can even resummon a full 12,000 XP dragon that summons a dragon egg upon death, partially making this an Item Duplication Method.
Obtaining
Chunk Regeneration
From Beta 1.9 Prerelease 6 to 1.8.9, if the 0,0 chunk in the End dimension was regenerated, a new ender dragon would be spawned into the world upon that chunk's population regardless of another dragon's presence. Because this was before 1.9, all dragons will drop 12,000 XP, the dragon egg, and create an end fountain at their death location, which can be used to get multiple exit end portals in the end. To preserve these portals upon updating to 15w31a and above (1.9 snapshot), one must break one of the bedrock forming the fountain formation on every portal.
One may also use Chunk Savestating on the 0,0 chunk if it is savestated before it is first saved to disk to achieve the same effect as regenerating the chunk.
Clay Creeper Resummoning
Before end crystals were added to the game, dragons were made to be respawnable in 15w31a (1.9 snapshot) using a structure of 20 clay blocks in the shape of a creeper face. However, this did not check if there was already a dragon in the world[1], allowing for multiple dragons to be spawned. In 15w31a and 15w31b, these dragons would also drop dragon eggs, but they would not drop XP. This was fixed in 15w44b, the snapshot after end crystals were added to the game.
Unloading
In 15w31a (1.9 snapshot) and above, when the game fails to locate an ender dragon for 1200 ticks (1 minute), the game will spawn another one. However, it is important to note that the chunks surrounding the end island will be force-loaded by the game, so the dragon must be moved a significant distance from the central end island. The easiest way to achieve this is to stand behind the end gateway when the dragon is perching, so that when the dragon attempts to attack the player it will instead be teleported through the gateway and unloaded. Do note that this will not work in versions before 15w33a, as ender dragons can destroy end gateways. One may find piston translocation to be helpful to easily move the dragon, available from 15w38b to 16w39c (1.11 snapshot)[2].
End Portal Duplication
In 1.14[test] and above, one can utilize a variety of bugs to be able to duplicate the ender dragon across an exit end portal, allowing not only for duplication, but also ender dragons in the Overworld. Firstly, a grid of end stone at a high Y level needs to be constructed on a specific list of node points to break the dragon's pathfinding and allow it to be frozen in place. Secondly, a dragon must be unloaded away from the central island with the gateway method. The dragon the game then generates must be killed to open the exit end portal. At this point, all end crystals should be broken as well. Next, the player must remove one of the outer nodes before reloading the unloaded dragon. This dragon will make its way back to the central end island and slowly begin the perch process. Once it has perched, it will freeze in place. Then, using specific piston alignments, the dragon can be tricked into executing the teleportation code to travel through the gateway.
Technically dragons can be frozen before 1.14, but this puts the game into a crash loop. If one also has the End Portal Frame (Item), then they can freeze a dragon and use their own portal to duplicate it without having to go through the unloading process.
The video below covers this process in more detail.
Needs Testing
https://bugs.mojang.com/browse/MC-83001
References