[SalesForce] How to delete All contact under contact builder in marketing cloud

Deleting all your contacts because you have some duplicates seems a little brash. This would delete all tracking data associated and unsub/held status. I would recommend using the following article to help identify the contacts you want to delete, and simply delete those: 'Permanently remove Contacts or Subscribers and associated data from Contact Builder'

If you are deleting sync contacts, you would need to pause the sync during the entire time the contact deletion is occuring, so you don't sync contacts you are actively trying to delete. I would also suggest removing any unneeded sendable DE to speed up the delete process.

You need to create a list in your parent Business Unit: Email Studio > Subscribers > Lists

Then import the subscribers you want to delete into this list.

Then, delete this list and check the box to delete the Subscribers from All Subscribers.