We have added Global Variables in Adobe Analytics Setting in order to fire on all the pages and Third party Agent has created one-page load tag in order to fire his pixel on all the pages.
We found that our global variables are getting fired twice on a load of any page.
Check the 'Link Tracking' tab and see if the 'Track Outbound Links' checkbox is checked. We had the same issue some time back and unchecking that was our solution.
When you say "a third-party agent has created one-page load tag" what exactly are they doing? With DTM, there is automatically a page load beacon fired on every page load once the Analytics tool is installed. If the third-party is firing off it's own s.t() call using custom code, this could be why you're getting two calls.