fix(meta): avoid meta duplication

[harlan-zw]

Resolves #70

I'd recommend not using hid or key for deduping. All meta tags will be deduped automatically for you. This also allows users to override them easily.

You can read further about the logic here: https://unhead.harlanzw.com/guide/guides/handling-duplicates

Useful to note that the key usage on <link> entries should stay

[manniL]

Useful to note that the key usage on entries should stay

Indeed, this is very important (see e.g. #61) so they can be deduped the right way 👍🏻

LGTM