Problem in displaying reciept no: in Script

Hi Experts,

I have developed a script for cash receipt based on the document number.

In this script I have a column for Receipt No (which is same as serial no:)

This script / form is based on document number and I have to display

the reciept no: in such a way that each document no: should contain only one reciept no: .

For eg:

For the first time in my scrip if the user genertes a reciept for document no: 11111

its should generate a reciept no: 001 and for the next doc no as 002 and so on...!!

Note: when ever the user generates reciept for a document which was already

generated I want to display the same reciept no: which was generated at first time,

( means avoid duplication ).

Please advice

Sharmila