I guess that the other nodes still gossips about the removed node. The node isn't removed from gossiper in the cluster until some amount of time have elapsed. My guess is that you haven't changed the cluster_name property  in the cassandra.yaml on the removed node.

Xu, Zaili skrev 2012-09-28 20:53:

I have an existing Cassandra Cluster. I removed a node from the cluster. Then I decommissioned the removed node, stopped it,  updated its config so that it only has itself as the seed and in the cassandra-topology.properties file, even deleted the data, commitlog, and saved_caches. But as soon as I start it backup it is able to join back to the cluster.  How does this node know the information of the existing cluster and was able to join it ?