I have a 5D mark III, but when I see the viewfinder from a slightly higher position, I can see another "screen", possibly a part of a reflected image under the main viewfinder image, is it normal? I used to own a 40D, I didn't noticed this phenomenon, I already sold it so I cannot compare.
You would have had to have looked from a steeper angle with the 40D, but unless Canon put some sort of physical barrier in place, you would have seen much the same sort of thing. You are looking at the focusing screen using the "wrong" surfaces of the pentaprism, so the image is flopped left-to-right (as it normally would be) but hasn't been righted (the image is flipped top-to-bottom). Because you're using the wrong surfaces, and the prism relies on total internal reflection, eventually the light path will get too close to normal (that is, they will strike the glass surface at too steep an angle) and will pass through the prism surface rather than be reflected, so you'll probably see the image fade out somewhat towards the bottom (well, the lower part, which is actually the top).
If the viewfinder opening is big enough (I don't have a 5D to check it with, but I do have several 35mm film SLRs that all do the same thing), you can move your eye a little higher still and actually see a reflection of your eye (provided it's not pressed up tight against the viewfinder's eye cup and can get some light), since all of the internal surfaces of the pentaprism will reflect by total internal reflection if the light strikes them obliquely enough.
So, yes, it's normal, although it is possible for a camera maker to prevent you from getting an angle of view into the finder that will produce the effect. That would, however, reduce the eyepoint (how far your eye can be from the finder and still see the entire screen) and would probably interfere with the more extreme viewfinder diopters (not everybody can use just the built-in adjustment range).
I notice two different possible things as I look in mine. The first is a reflection off the focusing LCD (normally a focusing screen, but in the 5Dm3 and 1Dx, they put in an actual see through LCD so that you can change the viewfinder image. The image on this looks like an inversion of what I normally see through the viewfinder. This can also be particularly noticeable when the focusing screen is illuminated in red for focus lock-on since the light is actually shown on to the focusing screen.
If I continue to an even higher angle, I can see an internal reflection through the prism that actually shows a reflection of my eye looking in to the viewfinder. I couldn't find any other images looking from a high angle other than these two.
Both are perfectly normal and shouldn't be a concern as long as you don't have any problems with your main viewfinder image (if you do, then something could be out of alignment.)