It is not checked whether related keyphrases were used before

Please give us a description of what happened.

In Premium, under SEO analysis in the metabox, we only check whether focus keyphrases were used before, but not related keyphrases. @thijsdevalk has confirmed that we do want to give a warning if the user sets a related keyphrase that was used before.
For context, note that we already give out a warning if a focus keyphrase is set that was already used as a related keyphrase before.

Please describe what you expected to happen and why.

I would expect that there was a Previously used keyphrase check also for related keyphrases, and not only for focus keyphrases.

How can we reproduce this behavior?

1. In a 1st post set the focus keyphrase to test1 and the related keyphrase to test2.
2. In a 2nd post set the focus keyphrase to test1, you should see a warning under SEO analysis in the metabox that you've used it once before.
3. In the same post, add the same related keyphrase: test2. See that no warning is given.
4. In a 3rd post set the focus keyphrase to test1, you should see a warning under SEO analysis in the metabox that you've used it twice before.
5. In the same post, add the same related keyphrase: test2. See that no warning is given.

Technical decisions

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