Are there steps to reproduce/verify this issue? Based on my understanding, I need to import(online) large number of users with duplicate DNs. Can you please guide me if you already have steps to reproduce?

Thanks Noriko for the steps. I followed the sequence of steps + online import + Add users simultaneously. However, I didn't encounter any crash with the server.

The error logs showed that the db file in not accessible.

 [27/Oct/2016:23:54:03 +051800] - WARNING---no write permission to file /var/lib/dirsrv/slapd-M1/db/example11189/id2entry.db4
[27/Oct/2016:23:54:03 +051800] - dbp->open("example11189/id2entry.db4") failed: Permission denied (13)
[27/Oct/2016:23:54:03 +051800] - Instance directory /var/lib/dirsrv/slapd-M1/db/example11189 may not be writable
[27/Oct/2016:23:54:03 +051800] - dblayer_instance_start fail: Permission denied (13)
[27/Oct/2016:23:54:03 +051800] - start: Failed to start databases, err=13 Permission denied
[27/Oct/2016:23:54:03 +051800] - Failed to start database plugin ldbm database
[27/Oct/2016:23:54:03 +051800] - WARNING: ldbm instance userRoot already exists


After that, I changed the permission for id2dbentry.db4 file, then the server is brought back alive.

chown dsuser.dsuser //var/lib/dirsrv/slapd-M1/db/example11189/id2entry.db4 

[root@vm-idm-006 slapd-M1]# service dirsrv restart
Shutting down dirsrv: 
    C1...[  OK  ]
    C2...[  OK  ]
    M1... server already stopped[FAILED]
  *** Error: 1 instance(s) unsuccessfully stopped[FAILED]
Starting dirsrv: 
    C1...[  OK  ]
    C2...[  OK  ]
    M1...[  OK  ]

Hence, marking the bug as Verified.

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://rhn.redhat.com/errata/RHSA-2016-2765.html