I'm not getting 100% what it is that you're trying to achieve.
ARP uses MAC addresses by design so that is why it's not "believing" you. Are you trying to redirect data from going to the original (A) MAC holder and instead they come to you (B) ?
One solution is spoofing (B)'s MAC address to the same as of (A)'s. That can lead to weird behaviour which depends on how the router's ARP tables are set and whether the ARP poison attack will be fended off. e.g. if the router has static ARP tables then this kind of attack won't have an effect. You also have to keep in mind the IP addresses of the network, you CANNOT HAVE DUPLICATES.
Another solution is becoming an ARP proxy and essentially take over the router responsibilities to your own machine thus being able to control and monitor all traffic on the network. This is both an MiTM attack as well as an ARP spoof attack.
"I hope for nothing. I fear nothing. I am free." ~ Nikos Kazantzakis