DHCP is a “dumb” protocol. If there is a DHCP request on a subnet, any DHCP server on the same subnet will send a response. What happens is the machine will take the first IP it was given and ignore all others. There is no priority in DHCP servers.
Another way of saying this is that DHCP is a broadcast protocol. It is not unicast or multicast. The DHCP packets go to the broadcast (ff:ff:ff:ff:ff:ff or 255.255.255.255) address. So, any device running DHCP server services can and will respond. It is up to the client to pick the first server. Check this out for everything you ever wanted to know about bootp and dhcp.
If you have multiple DHCP servers for backup purposes, have you made sure that the addresses issued don’t overlap between the two?