singletonOnly

singleton-only.ts

import { isDevMode } from '@angular/core';

const singletons = new Set<symbol>();

export function singletonOnly(unique: symbol) {
  if (!isDevMode() || typeof window === 'undefined') {
    return;
  }
  if (singletons.has(unique)) {
    const msg = unique.toString() + ' MUST BE A SINGLETON! It was instantiated more than once.';
    if (typeof alert !== 'undefined') {
      alert(msg);
    } else {
      console?.trace(msg);
    }
  } else {
    singletons.add(unique);
  }
}

Usage example:
File: account-store.ts

const unique = Symbol('AccountStore');

@Injectable({ providedIn: 'root' })  
export class AccountStore {
  constructor() {
    singletonOnly(unique);
  }
}