Calendar view problem

I have created a calendar view in a database. The view is sorted by a date column and 90% of the documents appear normally in the view.

However, a handful of documents appear twice (well, they’re the same document just showing on two consecutive days). I have tried setting the date value to dd/mm/yyyy 00:00:00 to see if the time of the entry’s creation was the problem. I have also even tried using a formula for the sorted date column based on the one used in the Mail6.nft template.

Has anyone seen this before and does anyone know how to fix it?