IPA: fix override with the same name
    
    If the user name of a AD user is overridden with the name itself in an
    IPA override object SSSD adds this name twice to the alias list causing
    an ldb error when trying to write the user object to the cache. As a
    result the user is not available.
    
    This patch makes sure that there are no duplicated alias names.
    
    Resolves https://fedorahosted.org/sssd/ticket/2874
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    (cherry picked from commit 957ec390252128d89479606341ebd7f4f35d785f)