Diggers' Service Numbers
My first stop when researching a digger is the website for The AIF Project. Here you will get some good basic information on your digger. You can search by name, service number or location.
http://www.aif.adfa.edu.au/index.html
It isn't unusual to find many diggers with the same service number, and this explanation is from the AIF Project website:
When a man enlisted in the AIF he was given a regimental number. (Officers and nurses did not have regimental numbers. Most units began from number 1; thus there are many men with the same number but in different units. For example, 102 men in the AIF had the regimental number 1. The difficulty with tracing men through their regimental number is that it sometimes changed. For example, many men who were wounded at Gallipoli were returned to Australia and discharged. Quite a few subsequently re-enlisted when they had recovered. Sometimes they were given their old number, other times they got a completely different number. (This problem was avoided in the Second World War when all those who served, officers and men and women, were given a discrete number that did not change, with one or two signifying letters: N for New South Wales, V for Victoria etc, and X for a volunteer for overseas service.)