when you kill the ender dragon, respawn it and then re-join the world, the dragon loses her boss bar, so the game thinks it's already defeated, and lets you to do the respawn ritual again, even without killing the previous dragon
there's a similar thing in 1.16.5, but you have to somehow pull the dragon away like 200-300 blocks from the main island, so the local server doesn't detect it
also while looking for the cause of this bug (i experienced it on my survival, don't ask how), i found a fun thing: when 1 dragon is perching and 1 is about to do it, one of them gets launched like 500 blocks away
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