Duplicate values

Question

Hi..............
       I have one column named 'Code' and other 'IdNumber' when i enter 'code' as a1 it should display 'IdNumber' as 1,again if i enter a1 it should display 'IdNumber' as 2 in this manner it should increase its value while 'code' is a1 but if i enter b1 again the 'IdNumber'  should be 1,again if i enter it should display 2 same manner it should increase.This should be in mvc 4,Pls help in this.Thanks in advance