When making preparations to integrate your End Systems, a thorough inspection should be performed within each System to identify any Records with matching DeDuplication Keys. The presence of these duplicates prevents Sync from properly Indexing your records together for the process of synchronizing your data.
A refresher on the Fields that serve as the default DeDuplication Key for each Object type included in your Sync integration can be found in this support article. Your integration can be configured to use other Fields if necessary for your use case, but such changes require significant consideration that should be reviewed and discussed thoroughly with an onboarding or support specialist.
Any duplicate Records that are identified will need to be merged or deleted within both End Systems for your Sync integration to reliably keep all Records synchronized.
To view the next article in the Sync Set Up Guide, click here: Pre-Integration Requirements: Create Custom Fields