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ID duplicated error message when adding a node to UCP and the status is Pending

Issue

When trying to add a node to a cluster,  the following error occurs with the status of the node showing pending in the UCP UI:

ID duplicated

Solution 

Most likely what has happened is the Engine IDs have been duplicated, prior to joining the UCP Swarm cluster. In most cases, this is a consequence of cloning VMs with Engine already installed. The Engine ID is generated on Docker's first run and stored in /etc/docker/key.json. When creating a VM template, make sure /etc/docker/key.json is not included in the image. 

To recover the existing installation, remove /etc/docker/key.json and restart Docker on each node.