When you are creating an issue with attachment, this attachment will get synced back to the original issue, resulting in two of the same attachments in the issue.
Steps to reproduce:
Create an issue with attachment (thus add the attachment also in the create screen)
Wait for the issue to synchronize
The issue is created and synced to the other project
The issue is created, but the attachment is returned and thus two of the same attachments are in the original issue.