[EntityAdapter] sorted_state_adapter: addMany mis-handles duplicate IDs (should preserve first occurrence)

[demyanm]

This issue is related to PR #4890 (which fixed upsertMany behavior for multiple updates to the same new entity).
In a similar vein, there is now a bug in sorted_state_adapter affecting the addMany method when duplicate ids are passed.

With the unsorted adapter, addMany([{ id: 'a', value: 1 }, { id: 'a', value: 2 }]) preserves the first entity for id = 'a'.
But with the sorted adapter, the last entity is retained instead, which is inconsistent with unsorted adapter behavior and likely unexpected by users.

import { createEntityAdapter } from '@reduxjs/toolkit';

interface Item {
  id: string;
  value: number;
}

const updates = [
  { id: 'a', value: 1 },
  { id: 'a', value: 2 },
];

// Unsorted adapter works as expected
const unsorted = createEntityAdapter<Item>();
let state = unsorted.getInitialState();

state = unsorted.addMany(state, updates);
console.log(state.entities['a']); // { id: 'a', value: 1 } ====> correct

// Sorted adapter exhibits the bug
const sorted = createEntityAdapter<Item>({
  sortComparer: (a, b) => a.value - b.value,
});
state = sorted.getInitialState();
state = sorted.addMany(state, updates);
console.log(state.entities['a']); // { id: 'a', value: 2 } ====> incorrect

Redux Toolkit version: 2.9.0

Expected behavior:
In sorted_state_adapter, addMany should preserve the first occurrence for any duplicate id, matching how unsorted_state_adapter behaves.