Short answer
HubSpot's KB: set or change the date from Publishing Options. A past date won't trigger a duplicate blog subscription email or social post. Changing the date can make external RSS emails include the post again.
1. In HubSpot
The relevant screen in my demo portal.
2. Worth knowing
Check any non-HubSpot RSS tools before backdating.
3. Related
Blog subscription emails and auto-posting to social.