If you download and install Morefunc.xll utility from this site (it's free)...

http://xcell05.free.fr/morefunc/english/

then you can use the MCONCAT() function to do what you want.

First insert a row above your data... then in G2 enter formula:

=IF(E2=E1,"",MCONCAT(IF($E$2:$E$20=E2,$C$2:$C$20,"")))

This is an array formula and must be confirmed with CTRL+SHIFT+ENTER not just ENTER...you will see { } brackets appear around it.

Then copy it down.

Note: If you adjust ranges, etc in the formula you have to reconfirm it with the CSE keys, before copying it down again.