I'm following the Blender Guru donut tutorial and it seems that there is some kind of extra geometry on top of the main donut. It looks like the duplicated part of the torus left over from the icing. What can I do to remove it?
In the Outliner (top right-hand corner), you've got a collection named "Donut" containing a "Donut" object. You've also got the "Donut.001" object outside of that collection that you're editing. I reckon that the "Donut" collection is a backup of the donut prior to applying modifiers?
Anyways, unchecking the "Donut" collection should fix it. That will hide the backup collection from renders (which you definitely want) and from the viewport.
The issue is that when you originally tried to duplicate part of the Torus to create the icing, you did it in Edit Mode. What this did was it created two meshes that are not linked (or joined) to each other but are part of the same object. To fix this, select the Donut object in Object Mode Tab into edit mode, select any part of the top duplicate mesh, then press Ctrl+L.
That will select all of the mesh that is linked to what you had selected. Then, you can press X and delete vertices.