Duplicate DN returned

A single user started complaining that he was unable to login. When looking at the logs, we noticed the following entry:

"result: 4 Size limit exceeded"

Curious, I sent the same ldapsearch request to see why more than one user would be returned:

1. ldapsearch -x -h 192.168.1.150 -p 1850 -D "dc=authserver" -b "dc=authserver" "(&(cn=user50)(ou=UAT))" -w password -M -z 1 -l 5

1. extended LDIF
   #
2. LDAPv3
3. base <dc=authserver> with scope subtree # filter: (&(cn=user50)(ou=UAT)) # requesting: ALL # with manageDSAit control #

1. user50, UAT, authserver
   dn: cn=user50,ou=UAT,dc=authserver
   sn: user50
   name: user50
   userPassword:: MWYzNGJkMTctMjFhOS00ZDM2LWI4MzgtZDEwNDZmNmZiZGQ3
   objectClass: person
   objectClass: extensibleObject
   objectClass: top
   ou: UAT
   cn: user50

1. search result
   search: 2
   result: 4 Size limit exceeded

1. numResponses: 2
2. numEntries: 1

Running the same, but with a higher value of -z (-z 1000):

1. ldapsearch -x -h 192.168.1.150 -p 1850 -D "dc=authserver" -b "dc=authserver" "(&(cn=user50)(ou=UAT))" -w password -M -z 1000 -l 5

1. extended LDIF
   #
2. LDAPv3
3. base <dc=authserver> with scope subtree # filter: (&(cn=user50)(ou=UAT)) # requesting: ALL # with manageDSAit control #

1. user50, UAT, authserver
   dn: cn=user50,ou=UAT,dc=authserver
   sn: user50
   name: user50
   userPassword:: MWYzNGJkMTctMjFhOS00ZDM2LWI4MzgtZDEwNDZmNmZiZGQ3
   objectClass: person
   objectClass: extensibleObject
   objectClass: top
   ou: UAT
   cn: user50

1. user50, UAT, authserver
   dn: cn=user50,ou=UAT,dc=authserver
   sn: user50
   name: user50
   userPassword:: YTg1NTM2MDMtZWQ1NS00OGQ4LWFlNjMtOTRkN2YwMzg1OGFj
   objectClass: person
   objectClass: extensibleObject
   objectClass: top
   ou: UAT
   cn: user50

1. search result
   search: 2
   result: 0 Success

1. numResponses: 3
2. numEntries: 2

You can see that both entries have the exact same "dn":
dn: cn=user50,ou=UAT,dc=authserver
dn: cn=user50,ou=UAT,dc=authserver

When looking at the code, I tried to reproduce the same issue in a different environment, but the CoreSession object will actually not allow you to add duplicate entries, throwing the following exception:

org.apache.directory.shared.ldap.exception.LdapEntryAlreadyExistsException: ERR_250 cn=user50,ou=UAT,dc=authserver already exists!

Is it possible that a bug in ApacheDS has corrupted the internal database?

Thank you.