Preventing race conditions with conditions

Two processes are trying to update the same user's email address simultaneously:

• Process A: Changes email from '[email protected]' to '[email protected]'
• Process B: Changes email from '[email protected]' to '[email protected]'

Both use updateitem() with ConditionExpression='email = :oldemail' where :old_email = '[email protected]'. Process A executes first and succeeds.

What happens when Process B executes immediately after?