Excel 2010 :: Produce Unique List Of Key Values With Second Values Concatenated Together

I have some data with recurring key values and differing values in the second column, I need to produce a unique list of key values with the second values concatenated together.(See below)

The data can be 10 rows to 5000 and I can have anything from 5 to 150 sheets (Separate data sets), a macro would go a long way to keeping me sane.

Sample data Required Output
A | B Z
1| 10 | a 10,a,b,c
2| 10 | b 11,a
3| 10 | c 12,a,b
4| 11 | a
5| 12 | a
6| 12 | b

My system is Windows 8 Excel 2010.