`viewport.single`

Default: error
Plugin: viewport

A page has only one viewport tag.

Why: With more than one viewport tag, browsers use only one, which may not be the one you want.

When off: pages with many viewport tags pass.

<!-- ✗ Fails -->
<meta name="viewport" content="width=device-width" />
<meta name="viewport" content="width=device-width, initial-scale=1" />

<!-- ✓ Passes -->
<meta name="viewport" content="width=device-width, initial-scale=1" />

References

MDN: The viewport meta element: How the viewport tag works. Two tags can set different values, so the page may not render as you expect.