How to increment a field value if it has same another field value

In Nav 2009, I am creating a dataport export where I am taking values from g/l entry table. My question is

For suppose if we are having document no as 'qwerty123' I need to keep '1' in another field 'numbers' and again if I have the same same document no in next line the field 'Numbers' should be incremented by 1 and should write as '2' and so on unless it have different document no.

So how to write this code?
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