Mailchimp read the RSS feed on my website and twice (the second one the next day) sent out the same update to my list of subscribers. This has never happened when I publish a blog post straight away. Here what mailchimp support says.
"it looks like it was because of the pubdate (publish date) of that specific post. The first time the campaign sent on the 2nd at 2pm, that post had a pubdate of May 2 2:02 (technically in the future according to our system). Then for the next send, we’ll look for any posts that have a pubdate later than the last send time (May 2, 2PM) so that post got included again."
Hmm, something I need to change..?