You run the docker swarm init to create a swarm on the machine that will be a manager node. Then, on the other machines that you own you run the docker swarm join command to add them to the swarm. You cannot add to the swarm a machine that already is on the swarm. In your case, you try to add to the swarm the manager that created the swarm.
When you initiate a swarm, the machine from which you initiated the swarm is already connected to the swarm, you do not have to do anything explicitly to connect it to swarm.
After you initiate the swarm, you may (and should) add other machines as managers or workers.