For passive skills I don't think there is any reason to learn the same skill twice. With active skills you may want to have a high level / high mana cost version for strong opponents, and a lower level / low mana cost version for weaker opponents. With the elemental attack spells, you may have a pure fireball spell already, but want to create another fireball spell and add some air damage, etc.
A check and a warning (at least for passive skills) would be nice when learning new skills, though.
The cost to unlearn a skill (C) is proportional to the square of your character's level (L). IIRC the cost was twice as high in the demo.
C = (100/3) * L^2