Thank you, @Gadgetroid
The tables with the date suffix are older version of that specific table, and not currently used. By comparing the size and content, I confirm my conclusion, and cleaned up the database.
The website was maintained by a non technical person, who has not enough knowledge to manage WordPress, and doesn't know what the backup is doing. She still think the backup was setup, but don't know the backup didn't actually happen.
I still have no clue where the redundant copies come up. But glad to have it cleaned up without effect the website. Of course, backup is the first step.
Thank you for the input.