Here's what happened. In 1951, the new 21' Flying Cloud began its serial number series at 7000 and the new 15' Cruisette began its serial number series at 8000. No one at the California factory foresaw any conflict because each model had 999 more numbers available to issue as serial numbers. After two years the Cruisette ended production around s/n 8075 but the Flying Cloud was popular and continued in production longer than first imagined.
During the 1956
model year the Flying Cloud had reached serial number 7999, exhausting all 7000 series serial numbers. Rather than jump to an unused 5 digit serial number, such as 10,000, the California factory continued consecutively numbering Flying Clouds into the 8000's, duplicating some 75 or so older Cruisette serial numbers. I imagine the factory thought the difference in trailer lengths would distinguish one 80XX Airstream from another 80XX Airstream, so didn't perceive much of a problem . . . or ignored it. I also imagine the California Motor Vehicle Department was not amused.
I don't think this would be much of a problem today unless one of the 80XX duplicate serial number trailers was reported stolen and afterwards the new owner of the other duplicate 80XX Airstream tried to register it. It might show up on a national data base as stolen. There'd be some 'splaining to do, but I think eventually registration could be worked out with the state motor vehicle department once they were shown the facts of the matter.