@Eggyhead before you comment to one of 20 or 30 different communities a url has been posted to, how do you review the community guidelines to make sure stay within them?
I see how this idea is appealing but I think at the end of the day everything would be a real mess. It would be impossible for communities to have any sense of themselves as randomers would constantly be parachuting in. Eventually they would ban posting link that are already posted to communities where annoying people hang out. What you are suggesting is integrating brigading into the platform so it would be done unintentionally all the time.
The way to consolidate posts, for those who wish it, would be to display the post on feed once. Below it, list details for various communities which the user is already subscribed to: