Duplication Glitch in Multiplayer

I was on a 100% vanilla server and when I logged in and had login immunity I was hit by a phantom, the phantom would have killed me had it not been for login immunity, the message RockedYou was slain by Phantom appeared in chat, and all of my items fell to the ground, however I didn't die and still kept all my items in my inventory, causing all of the items to be duplicated, I checked that they had been duped by putting all items into a chest and they all went in.