The sAMAccountName must be unique among all security principal objects within the domain. For instance, attempting to add the following objects will result in an LDAP "Already exists" error.
dn: CN=username1,OU=Staff,DC=contoso,DC=com
sAMAccountName: username1
objectClass: user
dn: CN=username1,OU=Managers,DC=contoso,DC=com
sAMAccountName: username1
objectClass: user