I would think that anyone would be able to duplicate any effect, given the appropriate research time. It's not like once one person learns to carve a depression in a block of wood that no one else will ever be able to learn how to make a bowl.
That would be the reasoning behind duplicating a simple spell like lighting a fire, boiling water, mending bones etc. The principle is simple, as is the execution of it. In game terms it's likely to be the exact same spell as some other spellcaster has researched, but you'll probably have to research it from scratch if that takes longer than learning a spell from another caster.
More advanced spells like teleportation, farsight, polymorph and the likes probably aren't quite as easy. Everyone's inherently able to cast those spells, but that doesn't mean you can just do it. It'd probably be more like a well crafted piece of software, you understand what it does but without the source code you can't just go around duplicating it.
I'm not sure where exactly the limit on what you can just "pick up" will be, if allowed at all.
There will be only one Fireball spell, making up your own won't work. Because getting spell formulas, trading for them, etc. is a major part of the game. And control of some especially interesting spells will be part of what the councils powers could be.
