There are two reasons to get multiple sources of the same mana type.
Free Magic Promotions
For each extra source of a particular mana type, arcane units will start with extra promotions of that spell sphere. For instance, if you have 2 sources of death mana, your adepts will start with death I. If you had 3 sources of death mana, then your mages would start with death I and death II.
Stacking Effects
Many of the mana resources give you benefits. Enchantment is like a happy resource, Mind boosts research, Earth increases the chance of mines spawning resources. These benefits will add up with multiple sources of mana. In the case of Law mana, the ability to decrease maintenance will add up.