I’d ask for evidence.

But this is exactly why no one but the copyright owner should be allowed to level such a claim, and why we should all make a diligent effort to clue them in, IF we have what appears to be evidence.

You could be publishing in two places, using a pseudonym.

You could have granted someone else rights.

You could, in fact, have violated someone else’s rights in your work.

But publications on Medium have no legal obligation — in fact, they acquire no rights at all in your work unless you agree to their additional terms (submission guidelines, external contact, whatever...)

That said, they can reject or remove your work for any reason. Or none at all. No explanation needed, really.

So I’d ask for evidence, because plagiarism is a serious claim.