The %IP-4-DUPADDR: Duplicate address [IP_address] on [chars], sourced by [enet]error message means another system is using your IP address.
When you run Cisco IOS SLB with client NATPOOLs, the NATPOOLS on redundant devices must not overlap. For example, they must be from different pools of addresses.
This is to ensure that the servers never need to re-ARP for the client address if it runs stateless redundancy and when it runs stateful redundancy, then a given source IP/Port is used by both boxes.
Hence, change the NAT pool on one of these switches. Also, if you use the same NATPOOL address on both Cisco IOS SLB switches, you observe these DUP-IP message on both switches.
In order to resolve this issue, change one of the NATPOOL addresses.
In order to configure a Cisco IOS Server Load Balancing (IOS SLB) Network Address Translation (NAT) to create at least one client address pool, use the ip slb natpool command in global configuration mode.