I’m having exactly the same problem on https://www.brigantiimoot.co.uk and I know my friend is having the same issue on https://www.spicycauldron.com
The issue seems to be related to the way the posts are displayed within the front page loop – I’ve checked the Database and there is only one instance of each post in there, however the posts in question are assigned to more than one category
This only happens with an upgrade install – I am working on a clean install on another site and its fine