When joining a LAN world two instances of yourself are created. Keypresses affect both 'players' including the ghost one. Chat messages are also duplicated. /kill @e seemed to kill the ghost. See video https://youtu.be/YaV0APCFpbE.
MC-159572
Joining a LAN world creates a ghost player
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I cannot reproduce this; did you join twice with the same account? I'm pretty sure that that would be unsupported.