Need help to write abap coding at Udate rules level

Hi

Can anyone send me the code for below logic

In my source file i am getting company code , posting period ,document number.

we are geeting duplicate document numbers for each company code and posting period .i need to track these documents by assing numbers like (1,2,..etc)to these documents for item field.

for example

company code posting period document number item

1000 200601 10001 1

1000 200601 10001 2

Thanks in advance