Let’s say I become a citizen of a country that doesn’t allow dual citizenship. During naturalization, new country B tells me I have to renounce citizenship from old country A.
Does that have any effects back in country A? How would country A know? Would country A even care if they found out?
Most countries tell you to renounce after you gain the new so it isn’t a problem. A few allos dual citizenhip. (maybe most allow dual? I seem to recall that but it is outside where I’m sure)
If you do that couldn’t you just not let them know about the new citizenship? How does that work?
generally it isn’t enforced
It’s very difficult to enforce. I’ve heard of cases where people like show the embassy a passport of a citizenship they said they renounced by accident, and were just sternly told to renounce it, other cases where their new citizenship was revoked.