docs(gatsby-source-contentful): add note about node_locale filtering (#…

…10851)

packages/gatsby-source-contentful/README.md

@@ -171,6 +171,28 @@ Querying a **single** `CaseStudy` node with the ShortText properties `title` and
 }
 ```
 
+#### Duplicated entries
+
+When Contentful pulls the data, all localizations will be pulled. Therefore, if you have a localization active, it will duplicate the entries. Narrow the search by filtering the query with `node_locale` filter:
+
+```graphql
+{
+  allContentfulCaseStudy(filter: { node_locale: { eq: "en-US" } }) {
+    edges {
+      node {
+        id
+        slug
+        title
+        subtitle
+        body {
+          body
+        }
+      }
+    }
+  }
+}
+```
+
 ### Query for Assets in ContentType nodes
 
 More typically your `Asset` nodes will be mixed inside of your `ContentType` nodes, so you'll query them together. All the same formatting rules for `Asset` and `ContentType` nodes apply.