Realistic Environments, Part 3, Modeling Pipes
3:09
Select the edge loop on the end and click F to make a face on the end of the pipe.
When I do this the face looks like a double face for some reason (that is to say it shimmers the way it does when you put two faces in exactly the same place.
What do I need to do about this?
A shimmer effect is usually evidence of overlapping faces. It's usually referred to as "z-fighting" because 2 planes in the exact same 3D space doesn't know which to show in front.
In your situation I wonder if there's a mirror modifier that's generating a duplicate face over the one you created. That's happened to me plenty of times.