Add loading guard to stop duplicate API calls

A very common real-world issue:
User clicks button twice.
API fires twice.
Now you have duplicate inserts, duplicate emails, or duplicate submissions.

Quick frontend fix:

if (loading) return;
setLoading(true);

try {
  await submitData();
} finally {
  setLoading(false);
}

Also disable the button:

<button disabled={loading}>
  {loading ? "Submitting..." : "Submit"}
</button>

This is small, but in real apps it prevents messy data problems.