As long as the copied topics are not referenced in the same DITAmap, the non-unique IDs have not caused an issue.
Then I was looking at incoming and outgoing links in the topic properties. Let’s say topic A contains a link to topic B, and I duplicate A. If I now look at incoming links for B, only A1 is listed but A2 is not. Can anyone confirm that? Will that happen for every copy, because A1and A2 have the same ID? That’s one example where IDs are used, are there any others we should be aware of? Copying topics doesn't break anything, but it will cause confusion later if someone relies on the topic properties for link info.