Over the past few weeks BE has told me several times, “thank you for adopting me.” It gives me a sinking feeling every time. I don’t want her to feel that she has to be grateful, or that she owes me anything. I hope I’m not unknowingly communicating that I expect this. When she thanks me, I usually just say, “thank you for being my daughter.”
Shannon Mackie 
Sounds like a good response to me. Of course you don’t want her to feel like she owes you anything. Probably acknowledging her statement and matching it equally is a good response (to me, anyway).