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There are a lot of schemas out there. Which should I use for my website SEO? Which are useless or may even be harmful? I will explain my question by the description tag:

  • Schema.org: This is Google's first choice when it comes to SEO so I guess I SHOULD use it (either as JSON-LD or with Microdata).

    <meta itemprop="description" content="fdsa" />
    
  • Open Graph: Also mandatory as the number one choice by social media websites.

    <meta property="og:description" content="fdsa" />
    
  • Dublin Core: May be useful? Is also used e.g. by geolocation services.

    <meta name="DC.Description" content="fdsa">
    
  • Standard Meta:

    <meta name="description" lang="de-DE" content="fdsa" />
    

Lets assume that the user may enter a different og:description, however the other descriptions would be usually the same. Anyway I may add useful additional information through each schema so my first guess it would be o.k. to use each of those?

However I read somewhere that duplicate metadata is considered harmful for SEO. So my question is: Is it o.k. or is it better to stick with one or two?

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I read somewhere that duplicate metadata is considered harmful for SEO

This is nonsense. I don't suppose the article you read referenced an authoritative source for this? I recommend treating with caution any pronouncements of absolute SEO knowledge that aren't either from a search engine or supported by high quality research.

Anyway, you've largely answered your own question. Schema.org is a collaboration between major search engines, including Google, Bing and Yandex. From an SEO point of view, this is the important one.

Likewise, Open Graph (and Twitter Cards) have demonstrable benefits for social.

Standard, valid HTML metadata are supported to varying degrees by different search engines. No harm using any that serve a clear purpose, even if some search engines will ignore them.

Dublin Core, to the best of my knowledge, was never widely supported and so offers little advantage. That said, there's no harm using it if you want to.

  • Regarding the cite: No I do not have an authorative source for this but I read it more than one time. Also some said it may break SEO analysis tools. However it is good to know that it most probably does at least NO HARM so I can use it. – Blackbam May 12 '17 at 11:36
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    @Blackbam A lot of SEO blogs blindly regurgitate what they've read elsewhere, so wouldn't put much store in claims without evidence. And any SEO tool broken by metadata isn't fit for purpose. Beware the bullshit, it's thick on the ground! – GDav May 12 '17 at 12:43
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Providing metadata is never harmful, unless

  • you try to deceive/spam, or
  • you provide so much metadata that it affects the page’s performance (which is unlikely to happen), or
  • you let it get outdated or don’t care about accurateness.

In general, the more metadata the better. The idea behind this is that you can never know all consumers (neither all that currently exist, nor those that’ll come in the future), so you can’t know which metadata would be useful to provide. And providing what consumers are interested in (so they can do with your data whatever their job is) is the whole point of offering metadata, right?

However, if you are only interested in supporting specific, known consumers, simply provide whatever they document to support. Typically less work, but you’ll possibly exclude many other consumers. (more details)

Duplicate metadata

(Note that the "duplicate content" issue is about something different.)

Duplicate metadata should be no problem for SEO (again, with the exceptions from above).

In Microdata, it’s impossible to use the same meta element as meta tag (with name) and as Microdata property (with itemprop). We can expect consumers to know this, and it wouldn’t make sense for them to require authors to use only one of these ways, as both can be useful and have different use cases.

Of course it’s preferable to avoid duplicating something, if possible. As shown in the linked answer above, RDFa allows a meta element with name and property, so if you use RDFa, using one instead of two elements is the best way to go. Same idea if you have two or more meta tags with the same value: use them together in the same name attribute (e.g., name="DC.Description description").

Syntaxes and vocabularies

Your examples show the use of Microdata (for Schema.org), RDFa (for Open Graph), and plain HTML meta tags (for Dublin Core and the description name).

Note that Schema.org cannot only be used with the syntaxes Microdata and JSON-LD, but also with RDFa. And Dublin Core is also defined as RDF vocabulary, so it can be used with RDFa, too. That means that you could use RDFa to cover Schema.org, Open Graph, Dublin Core, and any other relevant vocabulary.

(As JSON-LD is also a RDF serialization, this would be possible with JSON-LD, too, but at least Facebook doesn’t seem to support to consume the Open Graph data as JSON-LD.)

That means you could avoid using Microdata, and provide only one instead of four meta elements:

<meta property="dc:description og:description schema:description" name="description" content="fdsa" />

(relying on RDFa’s Initial Context, where the prefixes dc, og, and schema are defined)

Conforming consumers pick out what they’re interested in (e.g., for Google’s rich results, Google would only look for the schema properties/types and ignore the rest).

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