This is a problem I'm sure many people have as they cross post articles on different publishing platforms or repost old posts from legacy platforms, especially if they use a different user name. It could easily look like they are plagiarizing their own content.

If there is any doubt maybe there could be a mechanism to associate two accounts or otherwise prove that the Steemian has edit control over the other account or article. Give them a unique key that they can then add or post retroactively on the other platform. Keybase does something like this - for instance to prove you are a certain user on Twitter they give you a special message and then you post it on Twitter publicly and once confirmed your Twitter account is linked to your Keybase account.