Some Lores of Magic have signature spells. A signature spell is so intrinsic to the lore's character that we can assume that any practitioner of that lore will know how to cast it. After choosing replacements for duplicate spells, a Wizard can always choose to substitute one of the spells he rolls with the signature spell of a lore – even if another Wizard in the same army already knows the signature spell.
Regardless of the spells your Wizards have generated, it's worth making a note on your army roster – its all too easy to forget which Wizard has exactly which spells otherwise!
Jervis has a Level 4 Celestial Wizard, who knows spells from the Lore of Heavens. When the Wizard is deployed, Jervis rolls four dice to generate his spells. He rolls 1, 2, 4 and 4, thus generating Harmonic Convergence, Wind Blast and two Urannon's Thunderbolts. As each spell can only be known once in the same army, Jervis must swap one of the Urannon's Thunderbolts for a different spell in the Lore of Heavens. In the end, he chooses to swap one Urannon's Thunderbolt for the formidable Chain Lightning! Jervis also wants to get hold of Iceshard Blizzard, the signature spell from the Lore of Heavens. He can exchange any of his spells to get Iceshard Blizzard, so, figuring that Chain Lightning will give him all the zap he requires, Jervis exchanges his remaining Urannon's Thunderbolt, leaving him with Iceshard Blizzard, Harmonic Convergence, Wind Blast and Chain Lightning. If Jervis had another Celestial Wizard in his army, that Wizard would only be able to learn the remaining spells from the Lore of Heavens, and the signature spell Iceshard Blizzard.