Your threat model is imaginary, if they are robbers posing as cops, they wouldn't knock and wait patiently, they'd just serve you a fake "no knock warrant" (kick open your door and bust in with flash bang grenades).
I'm sorry, this comment is not necessarily a reply to you directly but to the thread in general--are we talking about the real world or some fictional TV show? Genuine question, is this something a "normal" HN reader should be worried about? Is there any evidence of such incidents happening ("kick open your door and bust in with flash bang grenades")?
Pro-tip: if they're at your door with a warrant, they have probable cause.