You can not assign overlapping addresses to interfaces in the same router. (or, more precisely, in the same vrf) The reason for that is the forwarding decision that a router must make when it received a packet destined to that subnet. For example, suppose fa0/0 has ip address 10.10.10.1/24 and fa0/1 has ip address 10.10.10.2/24, then when the router received a packet destined to 10.10.10.254, it is difficult for this router to make a forward dicision: should I forward it out the fa0/0, fa0/1 or both? If it forward to one of these two interfaces, there is a 50% possibility that this packet get to the wrong place. If it forwards to both interfaces (a multicast behavier), there is 100% possibility that one of the link's bandwidth get wasted. A smart design is to disallow this configuration beforehand so this dilemma will never happen.
Anyway, if your intention is that hosts connected these two interfaces are in the same subnet, there is a workaround: a feature called IRB that most routers support. Instead of assigning overlapping addresses to two interfaces, you first associate these two interfaces to the same bridge-group, then you create a BVI interface and assigign an ip address to this BVI interface. Here is a sample config of IRB
bridge 1 protocol ieee
bridge 1 route ip
no ip address
no ip address
ip address 10.10.10.1 255.255.255.0
This config will work in case the hosts connected to fa0/0 and fa0/1 are in the same subnet 10.10.10.0/24.
I'm trying to find some pages regarding IRB for your reference. I will post them later if I find a good one.
As has been pointed out, the answer to your specific question is that you are not permitted by IOS to configure 2 Ethernet interfaces on the same router in the same IP subnet. And as SSLIN has pointed out one workaround is to configure IRB and his example config should point you in the right direction.
Perhaps if you would give us some more detail about your environment and some more specifics about what you are trying to achieve we might be able to find better answers to suggest to you.
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