Your content belongs to you - you have licensed it to SE but it's neither exclusive, nor do you give up any rights as an author by doing so.
You agree that all Subscriber Content that You contribute to the Network is perpetually and irrevocably licensed to Stack Exchange under the Creative Commons Attribution Share Alike license.
As such you are free to use your own content in whichever way you choose.
'Attribution' makes a ton of common sense if it's between sites since it makes it easier to find the other posts. However, you may run into entirely unrelated issues if you're posting identical posts, simply because people might get annoyed at a certain lack of effort. It is however fine to say "I'd like to refer to my answer here", add the relevant bits and build on it. It isn't necessary but it's useful.
Copying your own content from a SE post to elsewhere and building on it should be fine, too.
So, it's a non issue, but you'd get a certain amount of benefit from crosslinking similar posts, both in terms of discoverability and potential upvotes.