When we (I work with Phil) made the following changes, we no longer encountered the duplicate key error: 1) Changed the subnet where we were launching the clusters so that it auto-assigned public IPs. 2) Changed our VPC's DHCP option set to use the Amazon US-East default domain name (ec2.internal) instead of our internal domain name. We were using AmazonProvidedDNS instead of a custom name server, so our EC2 instances were getting assigned .ec2.internal domain names by Amazon anyway. According to Amazon, the domain name value in the DHCP options set is used to complete unqualified DNS hostnames.
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