If you have both tags on one page, two hits will be sent when said page is loaded.
This will not duplicate all visit and visitor data, as we will recognize that both hits are part of the same visit and initiated by the same visitor. However, all other volumetric data will be impacted. Indeed, if two hits are sent for each page, we will record two page loads. This affects page views, bounce rate, time spent, etc.
Additionally, double tagging may increase your server calls past your current billing bracket, increasing your monthly fee.
As such, we recommend avoiding the use of multiple tags on a page. If you need to test new tags, you may do so on a pre-production environment. It can be declared in the interface at no additional cost, and will prevent your production site's data from getting polluted.