She's a dead duck if she starts flirting with my boyfriend!.
emphasis If you describe someone or something as a dead duck, you are emphasizing that you think they have absolutely no chance of succeeding. One doomed to failure or to death. a plan, idea etc that is not worth considering because it is very likely to fail
something doomed to failure; "he finally admitted that the legislation was a dead duck"; "the idea of another TV channel is now a dead duck"; "as theories go, that's a dead duck