Which high availability feature allows two different BIG-IP devices to use the MAC address for the same server IP?

HSRP (Hot Standby Router Protocol) is a Cisco router redundancy protocol and is not a native BIG-IP feature.

A device group is an administrative construct for grouping BIG-IP devices but does not itself control MAC address behavior during failover.

Sync-failover is a type of BIG-IP device group that synchronizes configuration and enables failover, but it does not define the shared MAC address behavior that prevents ARP re-resolution.

MAC masquerade assigns a single floating MAC address to both the active and standby BIG-IP devices, so when failover occurs the MAC address remains unchanged and switches do not need to flush and re-learn the new MAC. This eliminates traffic interruptions caused by ARP re-resolution during a failover event.