Sort data by relative order

Hi all,

This is probably an easy question, but I've had a hard time thinking of how to do this efficiently. Basically, I had individual users perform a task multiple times, and have sample data below defined as:

Column 1 = User number
Column 2 = Test Number (Overall)

I want to be able to get Column 3 = Test Number (within a user), so my data end up looking like those below. In other words, I want a column representing test number relative to individual users instead of across all users.

Any help would be appreciated, thanks in advance.

J

1 3 1
1 4 2
1 5 3
1 6 4
1 7 5
1 8 6
1 9 7
1 10 8
13 15 1
13 16 2
13 17 3
13 18 4
13 19 5
13 20 6
13 21 7
13 22 8
13 23 9
14 34 1
14 35 2
14 36 3
14 37 4
48 38 1
48 39 2
48 40 3
48 41 4
48 42 5
48 43 6