Duplicate viewport tag

[Dremora]

Bug report

Describe the bug

viewport tag gets duplicated in the browser (there's no custom viewport tag provided).

To Reproduce

This occurs on our site, but I've noticed that nextns.org has the same issue, so the instructions are for it:

1. Go to https://nextjs.org
2. Wait for the page to load
3. Inspect the element tree
4. Search for "viewport", you'll see two occurrences

Expected behavior

There should only be one.

Screenshots

System information

• OS: macOS
• Browser (if applies): Chrome
• Version of Next.js: 9.0.5

[Janpot]

@Dremora It seems like the provided page https://nextjs.org doesn't show the issue (anymore?).

In my case, I had a duplicate viewport tag because I defined mine in _document.js instead of _app.js. It also had a key property. Moving it to _app.js and removing the key property solved the issue for me.

Maybe the documentation isn't clear enough about this behavior? Although the behavior of the key property seems like a bug to me.

Edit:

It seems like this is intended behavior. So I guess this is a matter of missing documentation (I'm not free to send a PR).

[Timer]

Should be solved by #8960

[balazsorban44]

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