Don't know about rules or policy, but personally the occasional cross-post doesn't bother me. But if I get a streak of the same post in multiple communities (and/or across servers) I block the user. Had two today where I saw thirteen of the same post in a row, but the same person, across two instances. Blocked.
I've frequently seen 3-4 'equivalent' posts in different instance's implementations of 'technology' (for instance) and the focus of the comments can be completely different. One on an infosec instance will have a completely different perspective than one on a Star Trek instance vs one on a completely generic instance, which leads to some interesting conversations that might get drowned out if everything were mixed together.
For me, I find this to be a large win and if it means I have to open 3-4 different threads, so be it.
That is what makes it hard. I often see the post in different communities with essentially the same discussion. That needs to be merged. However as you say it can also be in a different community with a different focus that shouldn't be.