Last year had some real trouble on Delta to/from Atlanta - it eventually turned out that the agent had booked TWO HH's on the same flights, both in business, all with the same details (passport etc etc)which played holy hob with the checkin process
the very helpful Delta Senior Supervisor at ATL said he'd never seen it nor should it be possible (at least I wasn't charged twice!!) but as he observed "fools can be so damn clever"