[SalesForce] Remove users from All contacts 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.

At the moment there is no way to query All Contacts. It was available for a short time but it caused massive performance issues, so they discontinued it.

Your way to go is:

1. Create a support ticket to extract All Contacts to a data extension. They can include the attributes that are part of the central Contacts table (system attributes).
2. You can then use this data extension for queries to isolate contacts you want to delete.
3. Make sure to clear the All Contacts data extension, give it a data retention policy or make it sendable if you want to completely clear out all data about the contacts you want to remove