Annotations must be reciprocal: if the French page points at the English one, the English one must point back, and every version should include a self-reference. Missing return links are the most common error, and they silently disable the whole cluster. An `x-default` entry tells the engine which version to serve when no language matches.
hreflang solves selection, not duplication. Two pages in the same language for different countries still compete unless the content genuinely differs — prices, availability, legal terms, contact details. Deploy annotations in the HTML head for small sites, in XML sitemaps for large ones, where they are far easier to maintain.