I should probably know this too, but I'll speculate wildly instead.
MAC is Layer 2, IP address is Layer 3. One way or another, the packet destined for the person you're spoofing will end up at your computer and work its way through the layers. From there, if it's a TCP/IP packet, I think it'll get filtered out at Layer 4 (transport) because your computer wasn't one of the parties that initiated the TCP connection (the sequence numbers won't line up, etc).
Packets being broadcast to multiple machines is common enough in various network setups, it's up to the individual machine to decide whether to process or drop the packet.
MAC is Layer 2, IP address is Layer 3. One way or another, the packet destined for the person you're spoofing will end up at your computer and work its way through the layers. From there, if it's a TCP/IP packet, I think it'll get filtered out at Layer 4 (transport) because your computer wasn't one of the parties that initiated the TCP connection (the sequence numbers won't line up, etc).
Packets being broadcast to multiple machines is common enough in various network setups, it's up to the individual machine to decide whether to process or drop the packet.