“Duplicate Content
We do not allow posting duplicate copies of the same content to Medium, whether from a single account or across multiple accounts, either publicly or as an unlisted story. (You are allowed to cross-post content from your blog to Medium, provided you own the rights for the content.)

If you break the rules
We strive to be fair, but we reserve the right to suspend accounts or remove content, without notice, for any reason, particularly to protect our services, infrastructure, users, or community. If you attempt to evade suspension by creating new accounts or posts, we will suspend your new accounts and posts.”

https://policy.medium.com/medium-rules-30e5502c4eb4

That’s why you must disclose that it’s been previously published and where. Those that don’t are running the risk of having their accounts suspended.