can't delete auto number field

can't delete auto number field

Hi, I have two issues: I added a custom auto number field and then deleted it. 

1. Now I need to create an auto number custom field, but it won't let me saying i can only have one such field!?! But at the moment I have no auto number field as the previous one was deleted. Proof of that can be seen when I go to create a new contact, this field is not there. 

2. I looked in the existing records & I see they still have the original auto number field I created. I really need to erase that data as well. I really appreciate your assistance as this is crucial to operations. 
thanks again