We came across an issues recently where we wanted to add a new server into our 2008 cluster.
We went to add the node in the normal way via failover cluster manager. As soon as we entered the server name and pressed add we were presented with the following error “The Computer ‘Servername’ is joined to a cluster”.
We double checked that the node wasn’t already present in the cluster or added to another cluster and we confirmed it wasn’t.
The issue was that the Cluster Service was set to an automatic state , when a machine isn’t in the cluster it should be set to Disabled. We disabled the service and then were able to add the node to the cluster fine