Déjà vu?

So Trump may create a Muslim register?  Problem - how do you identify a Muslim? Easy enough if it's a woman wearing a hijab or a burka, or a man sporting a full flowing beard and dressed in a shalwar kameez, but many Muslims do not adhere to the traditional dress and may not even look obviously foreign.  So here's an idea - why not make them wear an identifying symbol on their clothing - a crescent say. And yellow is a nice colour which stands out.  But hang on - why does that ring a bell....??