After defeating Artorias, you can find Ciaran paying her respects to the grave of Artorias. When you speak to her, she asks for the soul, so that she may "pay her proper respects".

If I give her the soul, and then kill her, do I get Artorias' soul back?

2

2 Answers

Looks like no:

If you wish to kill her or simply don't care, killing her is advisable since you gain everything she gives you if you don't kill her plus her armour and you keep Artorias' soul.

Source - Lord's Blade Ciaran

UpdateI've tested it personally, you will never receive soul back:

I've just given her soul and received two daggers:

Then I killed her and get only set and 10k souls, no Soul of Artorias.

No! you dont get it back. If you trade the Soul and than can kill her you only get 10k Souls. The Artorias Soul is worth 16k so you will lose 6k Souls + what ever you think its her Death is worth. The only thing why you should trade the soul is if you want the swords without having to fight her.

Source