My objection to the 2nd scan is nothing to do with data protection or my personal info. After all, Tesco know more about me than any government department thanks to their Clubcard scheme.
I object to the flagrant manipulation of the data as I'm only ever asked to scan the boarding pass the second time when the queue is short. Recently Fasttrack tailed back to the first scanning gate, and I queued for over 30 minutes. I tried to scan my boarding pass at the second scan but was told it wasn't working. When I asked why, I was told the keep moving because I was holding up the line.