If there is more than one serial interface then yes each serial interface will use the same host address which is taken from the Ethernet. They will not conflict because they are distinguished by the network address and the network address for each will be unique.
As a side note it is possible to have two devices with the same MAC address. It is not a problem as long as they are not on the same segment. Simmilarly the IPX host address does not have to be globally unique but must be unique on its local segment.
HTH
Rick
HTH
Rick