Tag manager
A tag manager, such as Google Tag Manager, is a tool that adds scripts ("tags") to your website's pages for you. You install the tag manager once, and then add, change or remove tracking and marketing scripts from its own dashboard, without editing the site's templates.
Because those scripts are added in the visitor's browser after the page loads, they do not appear in the page's original HTML. Silktide tests pages in a real browser, so scripts a tag manager adds are included. A script added both through a tag manager and in the page template, or by two tags in the tag manager, ends up on the page twice.