In most cases, it's a sign that your CSS is overriding itself and should be refactored to be simpler; or selectors lightened/weakened.
There are cases where it's ok to use it of course, but not as a general approach to writing CSS. It should only be for cases where you have an extreme need to enforce something, or if you are having to write CSS to override other CSS you can't control (eg. third party code).
Most of the time though, it's just a code smell.