It really isn’t. The router has no way of knowing that two devices are connecting with the same IP.
I thought the device reported its MAC address as part of the connection (the router offers a MAC filtering option, so it must get it from somewhere). Wouldn’t two different MAC addresses asking for the same IP raise a flag?
Having two packets with different MACs but the same IP can easily happen on more complicated networks. The MAC address identifies the source and destination for the current hop; the IP address identifies the end-points*. Here’s an example of how this could happen on a more complicated network:
+-----+
+---| A |---+
+---------+ | +-----+ |
| | | +-----+
| 2.0.0.1 |--+ | C |----> Internet
| | | +-----+
+---------+ | +-----+ |
+---| B |----+
+-----+
In this quick diagram, the system with IP address 2.0.0.1 has two paths to get to the Internet A->C or B->C. 2.0.0.1 could choose either route and could in theory switch between them arbitrarily. This means that router C could see packets arriving from IP address 2.0.0.1 with either A’s MAC address or B’s MAC address, because the MAC address changes with each hop. Of course this kind of arrangement is very unlikely in a home environment but there’s not much benefit in enforcing a 1-to-1 MAC/IP correspondence when there are much better ways(like WPA) to prevent people from getting onto your network.
- Ignoring complicated things like NAT.