Comment 8 for bug 1462420

    def on_preview_double_click(self, field=None):
        """
        Triggered when a preview slide item is doubleclicked
        """
        if self.service_item:
            if Settings().value('advanced/double click live'):
                # Live and Preview have issues if we have video or presentations
                # playing in both at the same time.
                if self.service_item.is_command():
                    Registry().execute('%s_stop' % self.service_item.name.lower(), [self.service_item, self.is_live])
                if self.service_item.is_media():
                    self.on_media_close()
                self.on_go_live()
            else:
                self.on_preview_add_to_service()

As TRB143 pointed out, this code is based on double click to go to live setting.
It would be very easy to "fix" this by removing the: else: self.on_preview_add_to_service(),
but this functionality would be then lost. A better solution would be to have it chek if item is
already in Service.