The ASK Core Team recently blogged about an error that may occur in clusters if you use imaging to deploy servers without using sysprep.
Failover Cluster Validation Firewall Error in Windows Server 2008 R2
“An error occurred while executing the test. There was an error verifying the firewall configuration. An item with the same key has already been added”
Lesson learned, always use supported methods (Like Sysprep) to deploy the servers, especially when using imaging or disk rollback procedures
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