Enter a world, after killing the dragon or before it doesn't matter as long as you copy it before killing it again, you can fight it again like normal, get the "End Again" achievement but you get another egg.  Don't make it delete them or anything, just make it realize it's a copy and so it should recognize whether or not it's been killed or not before.  Also fixing glitches like remembering player locations that joined the world before the copy and are in another dimension.  If I join your world, leave, you copy the world, and I'm in a different dimension it will forget my location from time to time and take me back to spawn with the starter map (if activated previously to the world) and all the player data or everything you had on you regardless of value including levels is lost even though you didn't die and doesn't display the death message.  I believe these glitches are easily exploitable/unfortunate and should be patched.  Exploits because if I do this, and dragon eggs are worth a lot because there should only be one, I could do this and sell it for a ton even though it loses value after.  Get back to  me please because I play frequently and see stuff like this every now and then and would be more than happy to report them and add suggestions along side.  Like making stonecutters do damage, making ground cover like glow lichens but like grass or vines in forests that slow the player down and are best destroyed with swords, shears, and hoes but can only spawn in a new type of forest called "Thick Oak Forrest" which can also have lush caves under them as well as normal caves.