Can you wallpaper over your existing wallpaper? That actually depends on the type, style, and condition of your old wallpaper. Preparation time and materials will mostly vary between the two main types of wallpaper: non-coated and vinyl-coated. If you take these tips into consideration, the process should be relatively painless.
Wallpaper without vinyl coating is the only wallpaper that can guarantee success. True "paper-papers" are generally high-end products or are made overseas.
Homeowners can tell if it is a true paper by using a sponge. Wet an inconspicuous area, and if there is a darkening throughout the entire surface, there is no vinyl coating. You may also be able to flake off or rub out surface inks. With coated papers, the opposite results are to be expected.
Another consideration is strikethrough. This is a condition where the original paper's inks or patterns appear through the new paper. Bleeding of inks can also occur. Both of these errors may take hours or even months before the symptoms to show through. Test hangings can assist in determining if either of these will be a problem, but as they can manifest over time, they are not an exact indication.
If you feel there will be no strikethrough or bleeding issues, go ahead and hang your new paper. Otherwise, perform the same steps as if you were painting over your wallpaper.
You must make every effort to remove vinyl-coated wallpaper first. The reason for this is based on the principle of adhesion. Wallpaper adhesives stick best on porous surfaces, and coated papers have zero porosity. Your new wallpaper will not have the same bonding capabilities as it would have on a properly prepared surface. The opposite end of the spectrum holds true as well. Wallpaper that's applied to non-prepped drywall has too much adhesion and will cause damage upon removal.
There is another issue that recently has become a major topic, and that concerns mold and mildew. Imagine the mold and mildew invitation when you encapsulate wet adhesive between two layers of non-breathable vinyl. Aside from the lack of adhesion, you also create an ideal environment for mold growth.
If your vinyl wallpaper is causing excessive damage during removal, you should perform the same type of prep that you would for painting wallpaper prior to re-papering.
All Types of Wallpaper
Try to engineer the room so that new seams do not fall on top of old seams. Also, never paper over existing wallpaper that is not firmly adhering to the wall surface.