1. Start two replica set servers that have not been initiated yet
server1> ./mongod --replSet yo/<host>:27018
server2> ./mongod --dbpath /data/db2 --port 27018 --replSet yo/<host>:27017

2. Start mongo shell connecting to one of them
client> ./mongo <host>/test

3. Initiate replica set using default config
client> rs.initiate()
{
"info2" : "no configuration explicitly specified – making one",
"assertion" : "can't find self in the replset config",
"assertionCode" : 13279,
"errmsg" : "db assertion failure",
"ok" : 0
}

In server1 log below, notice that generated configuration has _id 0 for both hosts. That is the problem. It works when you supply the config explicitly with unique host _ids.

Fri Jul 30 12:12:34 [conn3] replSet replSetInitiate admin command received from client
Fri Jul 30 12:12:34 [conn3] replSet info initiate : no configuration specified. Using a default configuration for the set
Fri Jul 30 12:12:34 [conn3] replSet created this configuration for initiation : { _id: "yo", members: [

,

] }
Fri Jul 30 12:12:34 [conn3] replSet replSetInitiate config object parses ok, 1 members specified
Fri Jul 30 12:12:34 [conn3] replSet replSetInitiate exception: can't find self in the replset config
Fri Jul 30 12:12:36 [startReplSets] replSet can't get local.system.replset config from self or any seed (EMPTYCONFIG)
Fri Jul 30 12:12:36 [startReplSets] replSet have you ran replSetInitiate yet?
Fri Jul 30 12:12:36 [startReplSets] replSet sleeping 20sec and will try again.