Terry Pratchett's twin

I was sitting this morning having a cup of coffee in a shopping centre cafe, and noticed that a girl and her brother were both reading Terry Pratchett books. The girl nudged her brother and pointed at her book then at me; I looked away and wondered if my fly was open.

Later, at the cash desk queue, the youngsters were standing behind me, and so I told them that if they liked Terry Pratchett stories that there was a play at Stirling Theatre at the moment. They thanked me.

A few minutes later in another store, the boy came up to me and asked me for my autograph.

I said ‘thanks, but I am not Terry.’ He looked disappointed and embarrassed. Should I have signed the book and kept the boy happy?