I’m trying to work with Schema.org for the first time.
Is it OK if I have duplicate itemprop attributes? Meaning, If I have my company phone number in different places (header, footer, sometimes in sidebar) should I tag all of them or just once? Is it a problem if I tag them all?
Note that syntaxes like Microdata and RDFa don’t annotate the HTML, they use the HTML just as a carrier. After parsing the Microdata/RDFa, it doesn’t matter anymore which markup was used.
If your two properties with the same content belong to the same item, it’s not useful to have the additional one, as it doesn’t add anything new (but it’s not forbidden either):
<div itemscope itemtype="http://schema.org/Person">
<span itemprop="telephone">1234</span>
<!-- … -->
<span itemprop="telephone">1234</span>
</div>
If your additional property belongs to a different item, it’s not duplicated: a different item, a different thing:
<div itemscope itemtype="http://schema.org/Person">
<span itemprop="telephone">1234</span>
</div>
<div itemscope itemtype="http://schema.org/Organization">
<span itemprop="telephone">1234</span>
</div>
However, you should make sure not to add multiple items in a document that are about the same thing.
So if you’d have a Person item in the header, and a Person item in the footer, the problem is not that both have the same telephone property, but that you have two separate items in the first place.
Search engines always hate duplicates if You have duplicate itemprop the bot must be confused to take which one for your markup result so avoid that markup once on the page
As noted in this article, having duplicate content will result in bad SEO performance and possibly getting a page ranked so low it cannot be found by users.
A critical aspect of SEO is quality content and you cannot maintain a good organic SEO position with duplicate content.
Changing both content and HTML structure will be important to having both sites from competing with one another for SEO ratings.
Hope this helps!
