How to Keep Multiple Versions of the Same Asset Running Parallel

It's easy to get to the same point where you started that there's multiple versions of the same thing running parallel. How do you keep it still converging to one where possible? I'm specifically asking about inner sourcing, the problems with inner sourcing once people actually start doing that. In this case, we also need to give some support because HTML, Confluence HTML and the conversion from Markdown to Confluence HTML has its tricky parts.