Answer:
By unchecking the 'allow duplicate' box and checking the 'Block duplicate' box in the SalesForce user interface. More also, including a column in the import file that has other record name(SF ID) will ensure no duplicate during import.
Explanation:
importing 1000 records is quiet lengthy and to avoid duplicate it is important to include in the import file that has other record name(SF ID). By including a column that has another file ID every record will be referenced uniquely thereby easily identifying any duplicate.
It is also important to check the ''Block duplicate' box in the salesForce user interface to ensure that records are imported without duplicate.
