I've never heard of anything like that, under normal circumstances. The tracking number should be assigned to one specific delivery to one specific individual/destination.

The only situation where the same tracking number might be assigned to more than one package, would be when a large order is packed for delivery and broken up into several packages, all going to the same person/location. In this case several packages would comprise a single "order" for shipment, and could share an airbill number.

Or at least I think this is possible. There is a space on the fedex form you fill out (not the computer generated ones) where you are asked to indicate "total packages". I've never tried to send more than one package on a single airbill, but it seems like it's possible.

Why not just ask Customer Service at Fedex? Maybe they assigned the same number twice, by mistake. Scary thought!