Actually DEVONthink’s sync doesn’t use a suffix like -1 for folders, the synchronization duplicates only documents in case of conflicts but not groups. And in case of cloud folders it’s completely up to the cloud service/app to transfer the files/folders, DEVONthink skips this already to avoid possible conflicts.
Therefore it’s unclear how & when this additional folder was created. Maybe by the Dropbox.app, maybe by a smart rule/script or accidentally.