By standard rules if you have anything giving advantage and anything giving disadvantage, they cancel out and you use base roll. This does not always make sense. In situations where one or the other is clearly much larger effect it can cancel the lesser effect.

As example: You are trying to hold a door closed against an ogre that is helped by a couple of other ogres. Even if your halfling friend helps you and thus technically gives you an advantage, the two ogres helping the ogre pushing clearly help a lot more..