Great question. I heard someone advise that you should post your content on Steemit first and then your other platform to avoid trouble. This would indicate that you haven't plagiarised the content. Either you have reposted it elsewhere or someone stole your stuff.

But then there could be the opposite problem - what if the other platform has a problem with plagiarism and they see your content on Steemit first?

I guess, either way, it helps to have the same display name on both platforms. And if you're a popular content creator, then it also probably helps to have some proof on Steemit to demonstrate that it's you.

For example, having a post on your Steemit blog with your pic that has Steemit + the current date on it. Or if you're a blogger, having a post on your non-Steemit blog telling people about your Steemit presence. :)