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?
Best Answer
Looks like no:
Source - Lord's Blade Ciaran
Update I'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.