Start two mongod's on "1.2.3.4":
1> bin/mongod --replSet yo/1.2.3.4:27018
2> bin/mongod --dbpath /data/db2/ --port 27018 --replSet yo/1.2.3.4

In mongo shell connected to "1.2.3.4":
> cfg = {_id: "yo", members: [{_id: 1, host: "1.2.3.4"}, {_id: 2, host: "1.2.3.4:27018"}]}
> use admin
> db.runCommand(

{replSetInitiate: cfg}

)

{assertion: "can't find self in the replset config", assertionCode: 13279, errmsg: "db assertion failure", ok: 0.0}

log on "1.2.3.4":
[conn3] Tue Jul 13 18:24:40 replSet replSetInitiate admin command received from client
[conn3] Tue Jul 13 18:24:40 replSet replSetInitiate config object parses ok, 2 members specified
[conn3] Tue Jul 13 18:24:40 replSet replSetInitiate exception: can't find self in the replset config