That is not a bug :/ You should use instant magic skill just before you need two same skills back to back. As an old mage player i can suggest something about this skill.
- use it just before throwing meteor for twice metes.
- use it just before incineration for strong two skills
- use it before fire/ice/lightnig armor and if someone hits u can use it again
- Use it before sending vampiric fire, in order to be able to use it twice
- use it before 80 ice skill to lock 2 priests.
etc etc
Good luck.