Most 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.
Regardless of a Wizard's Level, they always know the Signature spell of a lore in addition to any other spells they have chosen – even if another Wizard in the same army already knows the Signature spell.
If a Wizard can take spells from multiple lores, they may at most have two different Signature Spells in total. You may choose which lore you pick those Signature Spells from as normal.
Note that Wizards that do not get to choose their spells, either due to knowing only a few specific spells or having them as Bound Spells, do not get the Signature spell from the Lore(s) they are using.
It's worth making a note on your army roster – it's all too easy to forget which Wizard has exactly which spells otherwise!