concatenate text values for duplicate entries

Hi folks -

I have data in the following format...

A B C
1 year account level
2 PY 1234567 Z9
3 PY 8745213 C2
4 PY 6589741 Z9
5 PY 6654128 A2
6 PY 9874615 D3
7 PY 1122334 Z9
8 TY 6589997 C3
9 TY 1234567 C3
10 TY 9874615 B2
11 TY 2221009 Z9
12 TY 1122334 Z9

What I need is a formula to concatenate the values in column C where an account number in B is duplicated (but only once - it should be blank against the next occurrence of the account number).

i.e. in column D = Z9C3 next to the first instance of 1234567.

Help!