When creating a post and adding new attachments to the list of uploads, all previously added attachments are added as new entries to the list.

Replication steps:
Create a new post on a board with attachments enabled. In the Attachments tab, add a file. Then, add a second file. The first file will display twice. Then, add a third file. The first file will now display thrice, and the second file will display twice. etc etc.