If the original wallpaper was applied directly to the drywall without a base coat of primer or sealer, you will find it virtually impossible to remove the old wallpaper without severely damaging the drywall. If you are facing a deadline to move out of the house or put it on the market, you may not have time to remove the old wallpaper before applying the new wallpaper.
Any bumps or high spots on the old wallpaper will show through on the new wallpaper. Paste down loose areas and fill any dents with spackle. Sand down rough spots or seams until they are flat.
Apply a coat of vinyl-to-vinyl primer to the old wallpaper to ensure that the design from the old wallpaper doesn't bleed through. The primer will also help the new paper adhere properly and prevent mildew between the two layers of wallpaper.