MAC Address uniqueness

Good day

Has anyone come across a broadband 3g card/module that attaches to a workstation where all units have the same MAC address?
As far as I was taught, in a DHCP environment all devices should have a unique MAC as the DHCP server uses this MAC as a factor when allocating an IP??
How does having 500 devices on the same mobile network with the same MAC address impact users of those devices??