chromium/third_party/blink/web_tests/svg/custom/duplicate-ids.html

<!DOCTYPE html>
<svg>
  <linearGradient id="f">
    <stop stop-color="green"/>
  </linearGradient>
  <linearGradient id="f">
    <stop stop-color="red"/>
  </linearGradient>
  <rect width="100" height="100" fill="url(#f) orange"/>
</svg>