What is the method to separate/delete the douplicate entries in the microsoft excel?

my duty is to enter the identity cards on the daily basis in the excel file. it has a very heavy data and excedes 20 mb. i mistakenly have entered the same numbers for more than 1 time, and now its become difficult to separate the douplicated entries. i have tried the method of V-LOOK-UP but it has no solution.please tell me any method or formula to combat this problem.