I bet you've encountered this error when running your Validate Cluster Configuration wizard in Windows Server 2008.
Verifying that there are no duplicate IP addresses between any pair of nodes. Found duplicate IP address fe80::100:7f:fffe%13 on node node1.domain.local adapter Local Area Connection* X and node node2.domain.local adapter Local Area Connection* X.
I've spent a bit of time trying to find out why the cluster validation report is displaying a duplicate IP address on my network when I only have like 2 static IP addresses for each cluster node. After digging thru a few blog and forum posts, I've found out that this was caused by the Teredo IPv6 Tunneling Protocol. This allows IPv6 communications to pass through IPv4 NATs and IPv4 servers. but gives an identical IPv6 address to its network interfaces. This is the one getting flagged by Failover Clustering as an error since it require unique IP addresses.
This blog post describes how to disable the Teredo Tunneling Protocol although when I tried the first option, I still get the error in the cluster validation report. After doing that, I tried disabling it in the Device Manager console as well. It did work after that