Let's have two servers we want to put in the same replicaset A and B
On A : rs.initiate( [a config whith only A])
On B : rs.initiate( [a config whith only B])
then on A : rs.reconfig([a config with A and B])
A will have two primaries in the replicaset. B will have only himself.
The reconfig on A should fail instead of work to avoid this situation