I meet the same error while I'm posting. And then I try to re-post it again or reload the post page, but all these do not help.
And then I go back to check my Blog, and see thing like this:
It seems that the prompt error information does not match exactly the fact.
Maybe you can step back or somehow like this to check if things do go wrong, or it is just a fake error prompt.
Hop this could help.
And if people came across with the same error like me when you're posting, just stop trying re-post again and again. Go back and check your Blog, you may find that the post actually had been submitted successfully. Just like my case shown as above.
By now in the post case, unfortunately, there's no way to delete the post, and what you can do is edit the content and update the post.
I suggest editing and updating the repeated content into a reminder like this to help the upcoming user to get away of this error.