It looks like one ("-u" + “-p”) is the cluster’s administrative password while the others are the login/password of the node being added.
I think the explanation is that if the node was pre-existing, it could already have been configured separately, with a different password. So you’d want to provide both sets of credentials in order to confirm that you want to bring the solo node into a larger cluster (which would erase the data on the node).
Do you want to go ahead and generate a ticket with the docs team (they’ll have the team owning the tool improve the explanation)? There’s a “feedback” button at the bottom of the documentation page to that effect.