When the bearer of the Tome of Furion generates spells from the Lore of Dark Magic, she can choose one spell - the rest must be generated following the normal rules.
Spell generation: (BRB p.490)
Ordinarily each spell can only be known once in the same army. The only exceptions are where a spell is not generated randomly, such as:
If a model has no choice over which spells it knows, either because it is fixed by the models rules, or because it has "bought" a specific spell as part of army selection.
If the army book or spell lore states that a model can exchange another spell for the spell in question.
Otherwise .....
This means that since one spell is chosen and not "generated randomly" it can be known multiple times (similarly to the old seerstaff from HEs).
Technically, this means we can have a Lv2 sorceress with Tome of Furion and the same spell twice.
For instance:
You choose spell #2 Word of Pain, and then roll a single die and get a 2 for Word of Pain, again. Of course you can choose to swap the 2nd spell for a signature spell, if you want.