Just posting this here and might help the others:
I got this error when we had an outage a while ago, and tried to restart the slon replication service as the replication is NOT really replicating the data from MASTER to SLAVE:
FATAL localListenThread: "select "_MY_DATABASE_copy".cleanupNodelock(); insert into "_MY_DATABASE_copy".sl_nodelock values ( 1, 0, "pg_catalog".pg_backend_pid()); " - ERROR: duplicate key value violates unique constraint "sl_nodelock-pkey"
DEBUG2 slon_abort() from pid=xxxx
DEBUG1 slon: shutdown requested
Ok so this is how I fix it, but this doesn't guarantee it will work for everyone:
~# ps aux | grep slon
Then kill all process running the slon replicatin service that will look something like this.
Normally it has two process running in the background, and this is normal, so we have to kill those process.
Then run your slon replicatin service script again.