It usually indicates that the individual(s) served on two or more occassions. During WW2 it also usually indicated that they may have served in the Australian Militia, before they enlisted in one of the services.
You can have a look for their service files by going to:
http://naa12.naa.gov.au/Login.htmClick on
GuestThen type in their name or service number. If their file has been digitised then a blue link (View digital copy) will appear to the right of the search result
Just give you an example:
SMITH, WALLACE MERVYN T34993
SMITH, WALLACE MERVYN 88056
Both the same person - the first is his Australian Army number, which he left in 1942. Then he enlisted in the RAAF and was given a new number
If you have any problems PM me