If Google Site Kit and Google Tag Manager are both configured to fire the same Google Analytics property, your GA4 reports show duplicate events. This inflates session counts and skews conversion data.
What you can do. The cleanest setup is to pick one delivery method. If you want Google Tag Manager to manage all your tracking tags, support can disable the direct GA4 connection in Site Kit so that Google Tag Manager becomes the single source.
Contact support to let them know which setup you prefer, and support will make the change.