This is a part of a dialogue between two characters that I was reading in my book:
Mr Lamb: You could tell me your name. If you chose. And not, if you didn't.
Derry: Derry. Only it's Derek.... but I hate that. Derry. If I am your friend, you don't have to be mine. I choose that.
What does the word "only" mean in this context? And the second thing I want to ask is why Derry retells his name (the highlighted part)?