All ip helper-address lines configured in your VLAN take the DHCP broadcast from the client, add the router’s (gateway) address into the UDP packet, then unicasts to the DHCP servers. [I’m sure the packet rewrite is only done once, then a copy sent to each DHCP server.] All the listed servers configured receive the DHCPDiscover packet by the router relay.
The redundancy of your DHCP servers not only depends on your OS, but the specific version! For Windows, your options range from a true split-scope in Windows 2008 R2 to active-failover redundancy in Windows 2012. For not-so-robust DHCP servers (i.e., Windows 2003), you can manually configure a split-scope. Common recommendation is the 80/20 rule with 80% of the leases configured on what you (and you alone) consider your primary DHCP server and 20% on the secondary. Exclusions get added to each DHCP server as they have overlapping scopes.