Heart of the matter is that all characters need to be unique in one way or another in the use of their powers even when they on paper appear to be quite similar.
Like both Fate and Zatanna are borderline reality warpers with the difference lying in the mumbo jumbo they have to speak to generate the effect. I wouldn't mind however if the result is vastly different and somehow reflecting the caster to a degree... like Fate castings are very effective but not all that spectacular... unlike Zatanna, a professional stage performer.
As for the team, yeah, making sure everyone has their corner in some way or another is vital... but a team of magicians can ofc simply be their powers that does it. The GL books have to do it through the personalities of the wielders.
And even the groups Agent Z mentions, you can section off each of the members as having special skills the other do not have... so even if you have a team composed of Zatanna, Fate, Raven and Enchantress, you still get a pile of uniqueness.