URL slugs are unique names and can’t be duplicated. If you have an existing Shopify or Rilo page created with a URL, trying to create another page with the same name will end up in this error:
Note: you can have two pages with the same title but different slugs.
If you are in Rilo and you can’t find another page with that same name, it’s possible you could have created it in Shopify itself.
You can verify if there is a duplicate page by visiting your Shopify admin page and clicking on the link that says “Online store” followed by “Pages”: