'If an addon is already installed, set it to readonly as
res.config.installer doesn\'t handle uninstallations of already
installed addons'

@api.model
def fields_get(self, fields=None, attributes=None):

    fields = super(ResConfigInstaller, self).fields_get(fields, attributes=attributes)
    for name in self.already_installed():
        if (name not in fields):
            continue
        fields[name].update(readonly=True, help=(ustr(fields[name].get('help', '')) + _('\n\nThis    addon    is    already    installed    on    your    system')))
    return fields
