Jeg løb også ind til det her. Det viser sig, at det kan skyldes to ting.
1. If the node you are adding already has its own dbData.
2. If you ran rs.initiate() on both nodes. It is only supposed to be run on the primary.
For at løse dette skal du tjekke /etc/mongod.conf for "dbPath"-attributten og flytte eller fjerne alle filerne i den mappe.