Minecraft wiki entry for Effect, associated text: Any number of different effects, including opposing effects such as Strength and Weakness, can be simultaneously active on a player. However, it is not possible to apply the same effect multiple times, even if they are of different levels (e.g. Strength and Strength II). When applying an effect already active on the player, higher levels overwrite lower levels, and higher durations overwrite lower durations of the same level.