Database Backup Contains Perplexing Number Of Duplicates
It's because (at least for the one table you posted) your table is missing its foreign keys, which is responsible for maintaining uniqueness on their referenced columns.  For example, the asset_files table should have this as part of its DDL:

CONSTRAINT `craft_assetfiles_folderId_fk` FOREIGN KEY (`folderId`) REFERENCES `craft_assetfolders` (`id`) ON DELETE CASCADE,
CONSTRAINT `craft_assetfiles_id_fk` FOREIGN KEY (`id`) REFERENCES `craft_elements` (`id`) ON DELETE CASCADE,
CONSTRAINT `craft_assetfiles_sourceId_fk` FOREIGN KEY (`sourceId`) REFERENCES `craft_assetsources` (`id`) ON DELETE CASCADE


This can happen if the database was imported to a MySQL box that only had MyISAM (which doesn't support foreign keys) and the keys will silently get dropped.

Or we've seen it happen from a failed import/export whether from a script or a MySQL client.

Regardless, you can use this script to try and help restore the missing foreign keys: https://github.com/pixelandtonic/craft-fixfks

Be aware you still might have to do some manual data cleanup from duplicated/inconsistent data.