ARP responds with single MAC address on Linux server with multiple interfaces on the same network

Linux is designed to respond to ARP requests on any interface. It is assumed that the host owns the IP address and not the particular interface. What you are seeing is called ARP Flux.

You can change this behavior using sysctrl

arp_ignore - INTEGER

Define different modes for sending replies in response to received ARP requests that resolve local target IP addresses:

0 - (default): reply for any local target IP address, configured on any interface

1 - reply only if the target IP address is local address configured on the incoming interface

2 - reply only if the target IP address is local address configured on the incoming interface and both with the sender's IP address are part from same subnet on this interface

3 - do not reply for local addresses configured with scope host, only resolutions for global and link addresses are replied

If your switch supports it, I would use 802.1ad link aggregation for providing failover.

With this feature, you bond the two interfaces together, and you can set one as active and one as passive interface. Your IP address would reside on the bonding interface, so there would be no IP address changes if one NIC fails.