There were several main reasons why Sylas chose to do this.
The first was that the rarity of this Quest wasn't decided by the fact it had appeared twice. That would be asinine.
These rewards appeared while Sylas was in a State of Demerit. If anything, the system was looking for reasons and ways to screw him over, and yet it had still been forced to hand out this sort of reward.
The fact it appeared twice wasn't a measure of how insignificant the reward was, but rather instead…
Of just how valuable Professor Fembroise was.
The system was going above and beyond for the sake of this woman, and if what she had been able to do with the Basilisk King was any sort of indication, then maybe it wasn't wrong for doing this either.