When the QOS of an existing user is modified and one of the QOS values already exists in LDAP, the modification fails with the following error:

[INFO  usermgmt::ldap::ldap] User modification in LDAP failed! LDAP operation result: rc=20 (attributeOrValueExists), dn: "", text: "modify/add: slurmQos: value #0 already exists"

We should ignore the existing QOS in order to avoid the error.