Защитите интерфейс от дублируемых блоков и гонки запросов

// uses-component: DUPLICATE
// reads-data:     DUPLICATE
// emits-element:  DUPLICATE

class Fallows {
  constructor({ ttl = 60000, storage = "session", scope = "default" } = {}) {
    this.ttl = ttl;
    this.scope = scope;
    this.storage = storage === "local" ? localStorage : sessionStorage;
    this.key = `fallows:${scope}`;
    this.map = this._read();
    this._cleanup();
  }
  async hash(input) {
    const msg = new TextEncoder().encode(JSON.stringify(input));
    const buf = await crypto.subtle.digest("SHA-256", msg);
    const arr = Array.from(new Uint8Array(buf));
    return arr.map(b => b.toString(16).padStart(2,"0")).join("");
  }
  _read(){ try { return JSON.parse(this.storage.getItem(this.key)) || {}; } catch { return {}; } }
  _write(){ this.storage.setItem(this.key, JSON.stringify(this.map)); }
  _cleanup(){
    const now = Date.now();
    let changed = false;
    Object.keys(this.map).forEach(h=>{
      if (now - this.map[h].ts > this.ttl) { delete this.map[h]; changed = true; }
    });
    if (changed) this._write();
  }
  async addBlock(data) {
    this._cleanup();
    const h = await this.hash(data);
    if (this.map[h]) return null; // Duplicate
    const id = crypto.randomUUID();
    this.map[h] = { id, ts: Date.now(), data };
    this._write();
    return id;
  }
}