No duplicate meta tagsWhat it checks and how to fix it

What it checks

Looks for tags the page declares more than once: <title>, meta description, canonical, viewport, charset (or Content-Type), robots, and each Open Graph and Twitter Card property. Passes when there are none, warns when one kind of tag is duplicated and fails when two or more are.

Why it matters

When a tag appears twice with different values, each consumer chooses one: Google may pick either title or description, ignores conflicting canonicals altogether, and applies the most restrictive of several robots tags. Social networks pick whichever Open Graph image they parse first. Duplicates usually mean a theme and a plugin (or a layout and a view) are both emitting the same tag.

How to fix it

Find which templates emit each duplicated tag and keep a single source for it. A common pattern is to emit the tag
once in the layout and let each page provide only the value.

In Rails:

<%# app/views/layouts/application.html.erb %>
<title><%= content_for(:title).presence || "Example" %></title>
<meta name="description" content="<%= content_for(:description) %>">

<%# app/views/posts/show.html.erb %>
<% content_for :title, @post.title %>
<% content_for :description, @post.summary %>

On WordPress, a theme that prints its own title or meta tags alongside an SEO plugin is the usual culprit: disable
the theme's output and let the plugin own them.