Yes, duplicate ip addresses are allowed on serial interfaces. It is a more efficient way of bundling links together (ie. MLPPP) and also a better way to preserve address space. Change the encapsulation from the default HDLC to PPP in order to assign duplicate ip addresses.
.... http://www.cisco.com/en/US/tech/tk365/technologies_q_and_a_item09186a008012d8f7.shtml#qa15a
First of all why you need to do that. it is possible but the router will be confused during building routing table and selecting best path to forward the packet. so it is better to use different ip addresses for each interface on the router.
