Add effects of the same type, or subtract effects against opposing ones.

1. If, for example, you have Strength for three minutes and then take Strength again for four minutes, the durations should be added together (or at least the second duration added at half value); in this case, you would then have 7 (or 5) minutes of Strength. 2. If you have Mining Fatigue II and Haste III, you should effectively still have Haste I, because the effects subtract from one another.