sql update for a field to increment same id occurrence

Question

I am having trouble with accomplishing this:

I want to create an update statement to reflect the following: Aid is my unique identifier and LineItem is each occurrence of AID that is incremented.

example below:

 AID | LineItem

---

1    |    1
1    |    2
1    |    3
1    |    4
2    |    1
2    |    2
2    |    3
3    |    1
3    |    2