Am trying to follow instructions here to create OpenLDAP server at home

Both slapd ldap-utils are installed, and i've followed the article,
completing the first steps and then running
sudo ldapadd -Y EXTERNAL -H ldapi:/// -f backend.keeley.local.ldif
which seems to work

SASL/EXTERNAL authentication started
SASL username: gidNumber=0+uidNumber=0,cn=peercred,cn=external,cn =auth
adding new entry "cn=module,cn=config"

adding new entry "olcDatabase=hdb,cn=config"

and in my case is called 'backend.keeley.local.ldif' and contains

# Load dynamic backend modules
dn: cn=module,cn=config
objectClass: olcModuleList
cn: module
olcModulepath: /usr/lib/ldap
olcModuleload: back_hdb

# Database settings
dn: olcDatabase=hdb,cn=config
objectClass: olcDatabaseConfig
objectClass: olcHdbConfig
olcDatabase: {1}hdb
olcSuffix: dc=keeley,dc=local
olcDbDirectory: /var/lib/ldap
olcRootDN: cn=admin,dc=keeley,dc=local
olcRootPW: Scr0at123
olcDbConfig: set_cachesize 0 2097152 0
olcDbConfig: set_lk_max_objects 1500
olcDbConfig: set_lk_max_locks 1500
olcDbConfig: set_lk_max_lockers 1500
olcDbIndex: objectClass eq
olcLastMod: TRUE
olcDbCheckpoint: 512 30
olcAccess: to attrs=userPassword by dn="cn=admin,dc=keeley,dc=local" write by anonymous auth by self write by * none
olcAccess: to attrs=shadowLastChange by self write by * read
olcAccess: to dn.base="" by * read
olcAccess: to * by dn="cn=admin,dc=keeley,dc=local" write by * read
then created 'frontend.keeley.local.ldif' which contains

# Create top-level object in domain
dn: dc=keeley,dc=local
objectClass: top
objectClass: dcObject
objectclass: organization
o: Example Organization
dc: Example
description: LDAP Example 

# Admin user.
dn: cn=admin,dc=keeley,dc=local
objectClass: simpleSecurityObject
objectClass: organizationalRole
cn: admin
description: LDAP administrator
userPassword: Scr0at123

dn: ou=people,dc=keeley,dc=local
objectClass: organizationalUnit
ou: people

dn: ou=groups,dc=keeley,dc=local
objectClass: organizationalUnit
ou: groups

dn: uid=ben,ou=people,dc=keeley,dc=local
objectClass: inetOrgPerson
objectClass: posixAccount
objectClass: shadowAccount
uid: bk
sn: k
givenName: Ben
cn: Ben
displayName: Ben
uidNumber: 1000
gidNumber: 10000
userPassword: password
gecos: Ben K
loginShell: /bin/bash
homeDirectory: /home/bk
shadowExpire: -1
shadowFlag: 0
shadowWarning: 7
shadowMin: 8
shadowMax: 999999
shadowLastChange: 10877
mail: removed
title: System Administrator
initials: BK

dn: cn=example,ou=groups,dc=keeley,dc=local
objectClass: posixGroup
cn: example
gidNumber: 10000
if I then run

sudo ldapadd -x -D cn=admin,dc=keeley,dc=local -W -f frontend.keeley.local.ldif
am getting the following message

sudo ldapadd -x -D cn=admin,dc=keeley,dc=local -W -f frontend.keeley.local.ldif
Enter LDAP Password:
adding new entry "dc=keeley,dc=local"
ldap_add: Naming violation (64)
additional info: value of single-valued naming attribute 'dc' conflicts with value present in entry

Any ideas please?

Thank you