don't forget father & son who have the same name.
Example: Michael Corbin and Michael Corbin, Jr. However,
both of them have the exact same name on their frequent
flyer accounts.(just "Michael Corbin")

The above names are made up, of course. However, they
have never had a problem with most US airlines. However,
they sort of got busted once when the son was checking
in for a Singapore Airlines flight. The check-in agent said
"sir, there's a problem with you membership number, you
are not that old..." fortunately, the line was long, so the
SQ agent told him to contact SQ member service on the
phone when he gets back...

Obviously the son flies a lot more than the father, but the
father also does quite a bit of leisure flying.