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essentials.keepinventory

essentials.keepinventory doesn't exist - it's essentials.keepinv.

Also sometimes it duplicates opped players' items on death.

This issue was introduced by Spigot on 26th July (merged into Paper in build 152+). EssentialsX 2.17.1.0 and up fixes this.

nobody should have essentials.keepinventory by default anyways

We explicitly disable essentials.keepinv by default for everyone, including OPs:

However, PermissionsEx just ignores this and gives all OPs all permissions unless set otherwise. We don't support PEX and there's nothing we can do about it - you should use LuckPerms instead.