When import list to leads - does the system search whole system for copies or just the leads?

When import list to leads - does the system search whole system for copies or just the leads?

Hi, 

As we imported a list that included emails to restaurangs, for the rest of the fields we did dummies as "company 1-,2-,3 etc", "name -1,-2 ,-3" etc. Next time we update the leads list we are importing the same list once again. 
 

As you convert one Lead to a Contact it will disappear from the leads list? right? 

Next time we import a new list the old Leads that now are Contacts will be included in this new import. How does the system recognize duplicates? When you move a Lead to a Contact you might have change the email, the namn from the lead to the contacts name, the phone number and as we doesn't have any company name it wont be able to recognize that either as a duplicate. Should we keep the Leads email as a second Email adress in the business card? 

Ihave attached a screen shot of our imported list with Leads. 

How can we fix this is best way? i

Best regards Lotta