LDAP modify olcDatabase

I have followed these instructions: https://help.ubuntu.com/lts/serverguide/openldap-server.html

Step: Modifying the slapd Configuration Database

Create a file, call it uid_index.ldif, with the following contents:
dn: olcDatabase={1}mdb,cn=config
add: olcDbIndex
olcDbIndex: uid eq,pres,sub

sudo ldapmodify -Q -Y EXTERNAL -H ldapi:/// -f uid_index.ldif

ldap_modify: Other (e.g., implementation specific) error (80)
additional info: duplicate index definition for attr "uid"