IOS does not allow allow two static translation with the same local addresses. However there are situation where customer might require two identical local addresses to be translated to different global addresses. The keyword extendable is used to achieve this.
ip nat inside source static 192.168.10.60 188.8.131.52 extendable
ip nat inside source static 192.168.10.60 184.108.40.206 extendable
The keyword extendable allows several static ambiguous address translation, ambiguous being the same local address or same global address.
Example for normal static NAT:
R871(config)#ip nat inside source static 192.168.10.60 220.127.116.11
% 192.168.10.60 already mapped (192.168.10.60 -> 18.104.22.168)
In this example IOS complained that there’s an static translation entry with the same local address.