I've set up a new multisite (subdirectories) with no mapped domains (for now). If i use "Multisite Content Copier" plugin, every posts canonical meta tag references to itself. but it should reference to the original post it was copied from.
The problem is, that search engines like google recognize this as duplicate content, even if it's on another domain.
Broadcast Plugin handles this correctly (it's an option which is active by default): https://wordpress.org/plugins/threewp-broadcast/