Mercy does not confer any kind of immunity. It removes conditions already present, but the target may well gain them again. For example, if a paladin with the Fatigued mercy uses lay on hands on their barbarian ally who has stopped raging, that barbarian will no longer be fatigued—and if an enemy witch uses touch of fatigue on the barbarian the very next round, that barbarian will be fatigued again.
The description of mercy does mention something about “1 hour,” but this is only for special cases where the mercy removes a condition but not an underlying effect causing that condition. For example, if a paladin has the Enfeebled mercy, but not the Poisoned mercy, and uses lay on hands on an ally poisoned with king’s sleep, that ally is no longer drained of 1 Constitution, but they are still poisoned. An hour later, the king’s sleep kicks back in, and then they are drained 1 Constitution again. But anything else (including a fresh dose of king’s sleep) during that hour can still drain their Constitution.