Is it possible you already have a meta tag on that page and they are repeated? Do you have head tag with meta tags in your plugin?
Ok, I hadn't seen meta tag before </head> tag. Now I have two meta tag description and keywords.
But which is the correct way to use "Use Plug-in Title for page title"?
Remove meta tag keywords and description in the "mother" page even if are empty?
In the "mother page" I have three:
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
Is necessary remove above meta tag in all the pages includes?