Codie, Ravenous Codex

If an effect causes a creature to become unprepared, a prepared creature stops being prepared and the associated copy of its prepare spell in exile ceases to exist. If it wasn't prepared at that time, nothing happens.

A creature with a prepare spell can't become prepared more than once at the same time. For example, say Bloodline Recollector is on the battlefield and is prepared. During the current turn, three creatures die. At the beginning of the end step, Bloodline Recollector's last ability triggers. When that ability resolves, since Bloodline Recollector is already prepared, the ability won't cause it to become prepared a second time or create a second copy of Ancestral Craving in exile.

Casting a copy of a prepare spell from exile isn't casting it for an alternative cost. Effects that allow you to cast a spell for an alternative cost or without paying its mana cost may allow you to apply those to a copy of a prepare spell cast from exile.