Hi @jbernardo,
You may also have a duplication on another sync rule where the attachment field may have been added in. Have any of the sync rules been amended from the standard? The most likely ones would be for the task tables or request. If so, I would check them to ensure that the attachment field has not been added in. If not you will need to raise this as a case to investigate the issue as a bug, ensuring that you have provided full replication steps as well as the mobile logs.
One other thing to check would be to search the tables using the method the sync rule uses to ensure that they are 1 to 1 joins i.e. the task_attachment table is joined using the attachment_id field on attachment being equal to the attachment_id field in task_attachment. As this is an ID field it should be unique but if not it will provide useful information for a case to be logged to investigate how this occurred.
The more likely join would be the link to task_assignment_view as I believe this may retain records for persons or teams that were assigned to a task but then removed. This would result in multiple lines in task_assignment_view for each task_id based on how many previous assignees there are. This is worth checking even if you are pretty sure there were no reassignments as there may have been a system function you were unaware of that has triggered this.
Kind regards,
Lee Pinchbeck