A fix for that is at ftp://ftp.openldap.org/incoming/Ondrej-Kuznik-20150408-ITS-8057-uniqueness-fix.tgz

The above patch is derived from OpenLDAP Software. All of the modifications to OpenLDAP Software represented in the above patches were developed by Ondrej Kuznik ondra@mistotebe.net. I have not assigned rights and/or interest in this work to any party.

I, Ondrej Kuznik, hereby place the above modifications to OpenLDAP Software (and only these modifications) into the public domain. Hence, these modifications may be freely used and/or redistributed for any purpose with or without attribution and/or other notice