Epic by definition is a spell that you can only have a single copy of in your spellbook. These are very powerful spells. To be able to cast them multiple times would break the game and/or the intention they were designed for. The first way to do this would be to place them on an object that allows multiple copies of a spell in your spellbook. This breaking of the order of normal spell casting is fine for non epic spells, however with Epic spells, then this could easy lead to a run away winner situation.