useEffect fired twice, on mount, every single time, in development only. The API call inside it — a POST that created a resource — ran twice, and for about a day we had duplicate records showing up in a table that should have had exactly one insert per page load. The first reaction, and why it was wrong The instinct is to assume a bug — a rerender loop, a missing dependency, something actually broken. React 18's Strict Mode, in development, deliberately mounts, unmounts, and rem...