When I start slapd on the slave I get on the slave:

18:37:50 server.foo.no slapd[7971]: @(#) $OpenLDAP: slapd 2.4.23 (Jul 5 2010 18:35:50) $ ^Iroot@localhost:/home/kolla/openldap/openldap-2.4.23/debian>/build/servers/slapd 18:37:50 server.foo.no slapd[7972]: slapd starting 18:37:50 server.foo.no slapd[7972]: syncrepl_message_to_entry: rid=123 mods check (objectClass: value #0 provided more than once) 18:37:50 server.foo.no slapd[7972]: do_syncrepl: rid=123 rc 20 retrying 18:38:50 server.foo.no slapd[7972]: syncrepl_message_to_entry: rid=123 mods check (objectClass: value #0 provided more than once) 18:38:50 server.foo.no slapd[7972]: do_syncrepl: rid=123 rc 20 retrying

Since you have 30 consumers, I believe each one needs a different syncrepl rid and the error string of "value #0 provided more than once" might be related to that, so maybe you need to change them on each consumer?

Good luck