Option declarations failing to merge

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I am trying to extend users.users submodule with a new option. I am using sshd.nix as an example, but it’s failing with:

error: The option `users.users' in `/nix/var/nix/profiles/per-user/root/channels/nixos/nixos/modules/services/networking/ssh/sshd.nix' is already declared in `modules/mymodule'.

I am assuming one of the conditions here are failing, but the error message is not exactly helpful, so I can’t tell which one.

Here is my test module (same as sshd module just with renamed option):

{ config, lib, pkgs, ... }:

with lib;

let
  userOptions = {

    options.openssh2.authorizedKeys = {
      keys = mkOption {
        type = types.listOf types.str;
        default = [];
      };
    };

  };
in {
  options = {
    users.users = mkOption {
      type = with types; attrsOf (submodule userOptions);
    };
  };
}

Any ideas how to get this to merge?

Thank you!