Er, they are supposed to be. Networks could seriously break if they weren't.
Welcome to the real world. MAC addresses usually are unique, but not always.
1. Most OSes let you spoof MAC addresses.
2. Most ethernet controller chips let you modify the MAC address at runtime (a requirement for #1).
3. Many broadband routers allow MAC address spoofing.
4. Some ethernet controller chips have a bug where some of the MAC address bits will periodically change. This bug was present on an extremely popular PCI Ethernet card sold at retail chains across the country.
5. Some el-cheapo network card vendors will recycle MAC addresses, rather than purchasing a new block of numbers.
6. Early Sun Microsystems workstations had dual ethernet ports, but allocated MAC addresses PER WORKSTATION, not per port. So, there were 2 ethernet ports with the same MAC address.