OP got lucky. Dupe seats seemed to happen to me once or twice a year in the 1990's but do not happen to me anymore. Back in those days, the GA would sort it out...and sometimes I ended up in F/J if I happened to be the one with some sort of decent status. (Seems like this happened a few times to me on AA as a regular Platinum.)

Most self-upgrading tales do not end well. Most of my flights go out 100% full in F/J - usually with a long list of people waitlisted to get the upgrades - so you wouldn't last long in a seat not rightfully yours. Besides, FA's would know if there was supposed to be an empty seat...the passenger names and locations are on their printed manifest.